Dissertations / Theses: 'Proportional hazards(PH) model'

1

Fei, Mingwei. "A study of the robustness of Cox's proportional hazards model used in testing for covariate effects." Kansas State University, 2012. http://hdl.handle.net/2097/13528.

Full text

Abstract:

Master of Arts
Department of Statistics
Paul Nelson
There are two important statistical models for multivariate survival analysis, proportional hazards(PH) models and accelerated failure time(AFT) model. PH analysis is most commonly used multivariate approach for analysing survival time data. For example, in clinical investigations where several (known) quantities or covariates, potentially affect patient prognosis, it is often desirable to investigate one factor effect adjust for the impact of others. This report offered a solution to choose appropriate model in testing covariate effects under different situations. In real life, we are very likely to just have limited sample size and censoring rates(people dropping off), which cause difficulty in statistical analysis. In this report, each dataset is randomly repeated 1000 times from three different distributions (Weibull, Lognormal and Loglogistc) with combination of sample sizes and censoring rates. Then both models are evaluated by hypothesis testing of covariate effect using the simulated data using the derived statistics, power, type I error rate and covergence rate for each situation. We would recommend PH method when sample size is small(n<20) and censoring rate is high(p>0.8). In this case, both PH and AFT analyses may not be suitable for hypothesis testing, but PH analysis is more robust and consistent than AFT analysis. And when sample size is 20 or above and censoring rate is 0.8 or below, AFT analysis will have slight higher convergence rate and power than PH, but not much improvement in Type I error rates when sample size is big(n>50) and censoring rate is low(p<0.3). Considering the privilege of not requiring knowledge of distribution for PH analysis, we concluded that PH analysis is robust in hypothesis testing for covariate effects using data generated from an AFT model.

APA, Harvard, Vancouver, ISO, and other styles

2

Crumer, Angela Maria. "Comparison between Weibull and Cox proportional hazards models." Kansas State University, 2011. http://hdl.handle.net/2097/8787.

Full text

Abstract:

Master of Science
Department of Statistics
James J. Higgins
The time for an event to take place in an individual is called a survival time. Examples include the time that an individual survives after being diagnosed with a terminal illness or the time that an electronic component functions before failing. A popular parametric model for this type of data is the Weibull model, which is a flexible model that allows for the inclusion of covariates of the survival times. If distributional assumptions are not met or cannot be verified, researchers may turn to the semi-parametric Cox proportional hazards model. This model also allows for the inclusion of covariates of survival times but with less restrictive assumptions. This report compares estimates of the slope of the covariate in the proportional hazards model using the parametric Weibull model and the semi-parametric Cox proportional hazards model to estimate the slope. Properties of these models are discussed in Chapter 1. Numerical examples and a comparison of the mean square errors of the estimates of the slope of the covariate for various sample sizes and for uncensored and censored data are discussed in Chapter 2. When the shape parameter is known, the Weibull model far out performs the Cox proportional hazards model, but when the shape parameter is unknown, the Cox proportional hazards model and the Weibull model give comparable results.

APA, Harvard, Vancouver, ISO, and other styles

3

Ishak, Khajak. "Omitting a strong covariate from Cox's proportional hazards model." Thesis, McGill University, 2001. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=33005.

Full text

Abstract:

The primary objective of this thesis is to explore the effect of omitting a strong prognostic factor from Cox's proportional hazards model when analyzing data from randomized trials. The secondary objective is to provide an overview of the Cox model. We first present the properties of the model, the method of maximum partial likelihood and the elements available on which to draw inferences concerning parameters. We then discuss various methods used to assess the tenability of the proportional hazards assumption as well as ways to incorporate non-proportional hazards in the model.
In the third and final chapter, we address the primary objective of the thesis. In linear regression analysis, unbiased estimates of the effect of the intervention can be obtained even when important but balanced determinants of the outcome are omitted from the model; the precision of the estimates are improved, however, with the inclusion of strong covariates. The logistic and Cox regression (and other non-linear) models do not share this property, however. We discuss the literature on this topic and provide examples to illustrate the problem. We examine the situation for the Cox model in more detail with the analysis of data from an experiment on the effect of increased sexual activity on the longevity of male fruitflies.

APA, Harvard, Vancouver, ISO, and other styles

4

He, Bin. "APPLICATION OF THE EMPIRICAL LIKELIHOOD METHOD IN PROPORTIONAL HAZARDS MODEL." Doctoral diss., University of Central Florida, 2006. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/4384.

Full text

Abstract:

In survival analysis, proportional hazards model is the most commonly used and the Cox model is the most popular. These models are developed to facilitate statistical analysis frequently encountered in medical research or reliability studies. In analyzing real data sets, checking the validity of the model assumptions is a key component. However, the presence of complicated types of censoring such as double censoring and partly interval-censoring in survival data makes model assessment difficult, and the existing tests for goodness-of-fit do not have direct extension to these complicated types of censored data. In this work, we use empirical likelihood (Owen, 1988) approach to construct goodness-of-fit test and provide estimates for the Cox model with various types of censored data. Specifically, the problems under consideration are the two-sample Cox model and stratified Cox model with right censored data, doubly censored data and partly interval-censored data. Related computational issues are discussed, and some simulation results are presented. The procedures developed in the work are applied to several real data sets with some discussion.
Ph.D.
Department of Mathematics
Sciences
Mathematics

APA, Harvard, Vancouver, ISO, and other styles

5

Pelagia, Ioanna. "Variable selection of fixed effects and frailties for Cox Proportional Hazard frailty models and competing risks frailty models." Thesis, University of Manchester, 2016. https://www.research.manchester.ac.uk/portal/en/theses/variable-selection-of-fixed-effects-and-frailties-for-cox-proportional-hazard-frailty-models-and-competing-risks-frailty-models(c75c6314-f43e-4d69-a2de-942bece6a404).html.

Full text

Abstract:

This thesis focuses on two fundamental topics, specifically in medical statistics: the modelling of correlated survival datasets and the variable selection of the significant covariates and random effects. In particular, two types of survival data are considered: the classical survival datasets, where subjects are likely to experience only one type of event and the competing risks datasets, where subjects are likely to experience one of several types of event. In Chapter 2, among other topics, we highlight the importance of adding frailty terms on the proposed models in order to account for the association between the survival time and characteristics of subjects/groups. The main novelty of this thesis is to simultaneously select fixed effects and frailty terms through the proposed statistical models for each survival dataset. Chapter 3 covers the analysis of the classical survival dataset through the proposed Cox Proportional Hazard (PH) model. Utilizing a Cox PH frailty model, may increase the dimension of variable components and estimation of the unknown coefficients becomes very challenging. The method proposed for the analysis of classical survival datasets involves simultaneous variable selection on both fixed effects and frailty terms through penalty functions. The benefit of penalty functions is that they identify the non-significant parameters and set them to have a zero effect in the model. Hence, the idea is to 'doubly-penalize' the partial likelihood of the Cox PH frailty model; one penalty for each term. Estimation and selection implemented through Newton-Raphson algorithms, whereas closed iterative forms for the estimation and selection of fixed effects and prediction of frailty terms were obtained. For the selection of frailty terms, penalties imposed on their variances since frailties are random effects. Based on the same idea, we further extend the simultaneous variable selection in the competing risks datasets in Chapter 4, using extended cause-specific frailty models. Two different scenarios are considered for frailty terms; in the first case we consider that frailty terms vary among different types of events (similar to the fixed effects) whereas in the second case we consider shared frailties over all the types of events. Moreover, our 'individual penalization' approach allows for one covariate to be significant for some types of events, in contrast to the frequently used 'group-penalization' where a covariate is entirely removed when it is not significant over all the events. For both proposed methods, simulation studies were conduced and showed that the proposed procedure followed for each analysis works well in simultaneously selecting and estimating significant fixed effects and frailty terms. The proposed methods are also applied to real datasets analysis; Kidney catheter infections, Diabetes Type 2 and Breast Cancer datasets. Association of the survival times and unmeasured characteristics of the subjects was studied as well as a variable selection for fixed effects and frailties implemented successfully.

APA, Harvard, Vancouver, ISO, and other styles

6

Milner, A. D. "Detecting changes in covariate effect in the Cox proportional hazards model." Thesis, University of Newcastle Upon Tyne, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.239639.

Full text

APA, Harvard, Vancouver, ISO, and other styles

7

Oman, J. P. "Case influence in proportional hazards with an application in renal transplantation." Thesis, University of Newcastle Upon Tyne, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.287419.

Full text

APA, Harvard, Vancouver, ISO, and other styles

8

Andersson, Niklas. "Estimating Companies’ Survival in Financial Crisis : Using the Cox Proportional Hazards Model." Thesis, Uppsala universitet, Statistiska institutionen, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-225982.

Full text

Abstract:

This master thesis is aimed towards answering the question What is the contribution from a company’s sector with regards to its survival of a financial crisis? with the sub question Can we use survival analysis on financial data to answer this?. Thus survival analysis is used to answer our main question which is seldom used on financial data. This is interesting since it will study how well survival analysis can be used on financial data at the same time as it will evaluate if all companies experiences a financial crisis in the same way. The dataset consists of all companies traded on the Swedish stock market during 2008. The results show that the survival method is very suitable the data that is used. The sector a company operated in has a significant effect. However the power is to low too give any indication of specific differences between the different sectors. Further on it is found that the group of smallest companies had much better survival than larger companies.

APA, Harvard, Vancouver, ISO, and other styles

9

Pal, Subhamoy. "An Approach to Improving Test Powers in Cox Proportional Hazards Models." Bowling Green State University / OhioLINK, 2021. http://rave.ohiolink.edu/etdc/view?acc_num=bgsu1626893233789827.

Full text

APA, Harvard, Vancouver, ISO, and other styles

10

Cheung, Tak-lun Alan, and 張德麟. "Modelling multivariate interval-censored and left-truncated survival data using proportional hazards model." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2003. http://hub.hku.hk/bib/B29536637.

Full text

APA, Harvard, Vancouver, ISO, and other styles

11

Putcha, Venkata Rama Prasad. "Random effects in survival analysis." Thesis, University of Reading, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.312431.

Full text

APA, Harvard, Vancouver, ISO, and other styles

12

Thompson, Kristina. "An Introduction to the Cox Proportional Hazards Model and Its Applications to Survival Analysis." Thesis, Southern Illinois University at Edwardsville, 2015. http://pqdtopen.proquest.com/#viewpdf?dispub=1571931.

Full text

Abstract:

Statistical modeling of lifetime data, or survival analysis, is studied in many fields, including medicine, information technology and economics. This type of data gives the time to a certain event, such as death in studies of cancer treatment, or time until a computer program crashes. Researchers are often interested in how covariates affect the time to event and wish to determine ways of incorporating such covariates into statistical models. Covariates are explanatory variables that are suspected to affect the lifetime of interest. Lifetime data are typically subject to censoring and this fact needs to be taken into account when choosing the statistical model.

D.R. Cox (1972) proposed a statistical model that can be used to explore the relationship between survival and various covariates and takes censoring into account. This is called the Cox proportional hazards (PH) model. In particular, the model will be presented and estimation procedures for parameters and functions of interest will be developed. Statistical properties of the resulting estimators will be derived and used in developing inference procedures.

APA, Harvard, Vancouver, ISO, and other styles

13

Sasieni, Peter D. "Beyond the Cox model : extensions of the model and alternative estimators /." Thesis, Connect to this title online; UW restricted, 1989. http://hdl.handle.net/1773/9556.

Full text

APA, Harvard, Vancouver, ISO, and other styles

14

Wang, Wen-Chyi. "Regularized variable selection in proportional hazards model using area under receiver operating characteristic curve criterion." College Park, Md.: University of Maryland, 2009. http://hdl.handle.net/1903/9972.

Full text

Abstract:

Thesis (Ph. D.) -- University of Maryland, College Park, 2009.
Thesis research directed by: Dept. of Mathematics. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

APA, Harvard, Vancouver, ISO, and other styles

15

Imbayarwo-Chikosi, Venancio Edward. "Genetic evaluation of functional longevity in South African Holstein cattle using a proportional hazards model." Thesis, Stellenbosch : Stellenbosch University, 2015. http://hdl.handle.net/10019.1/98128.

Full text

Abstract:

Thesis (PhD)--Stellenbosch University, 2015.
ENGLISH ABSTRACT: Improvement and selection of superior animals for longevity is a viable complimentary strategy for increasing functional longevity of Holstein dairy cattle. Genetic evaluation of animals for functional longevity is a prerequisite for improvement of the trait. This study was carried out to determine non-genetic factors that influence functional longevity in South African Holstein cattle, as well as to estimate genetic parameters for functional longevity, estimate sire breeding values, and to establish the contribution of conformation traits to the risk of cows being culled from dairy herds. Analyses were carried out using records of 166 222 daughters of 2 051 sires from 1 545 herds in six regions of South Africa. The regions were the Western Cape, Eastern Cape, Northern Cape, Free State, Kwa-Zulu Natal and the combined Gauteng & North Western Provinces. Data were analysed using a piecewise Weibull sire-maternal grandsire model in which the baseline hazard was assumed to change at 270 and 380 days in milk as well as at drying-off. The fixed effects model comprised of the time-independent effect of age at first calving, the combined time-dependent effects of region x year of calving, number of lactations x within-herd milk production class, year of calving x within-herd protein and year of calving x fat percentage production class. Model also had the combined time-dependent effect of herd size x annual herd size change. The random herd-year and sire effects were later included in the model for the estimation of sire and herd-year variance components. Analyses indicated a decline in the survival function, an indicator of functional longevity, among cows that calved for the first time in 2000, 2004 and 2008. Cows that delivered their first calf in 2000 had better survival functions that those that calved for the first time in 2004 and 2008. All fitted effects significantly contributed to the risk of a cow being culled from a herd. Within-herd milk production made the largest contribution to the risk of a cow being culled from a herd. Survival was best in the Northern Cape and worst in Eastern Cape. The risk of being culled was the highest for cows in the second stage of lactation (271- 380 days), when the entire lactation period was considered. High producing cows were more likely not to be culled from a herd than poor producing cows. Cows were more likely to be culled for low protein production percentage than within-herd fat production percentage. The risk of being culled was low for heifers calving at 20 to 25 months of age. The higher risk of culling among cows with multiple lactations indicated the culling policy of dairy farmers to retain a higher number of younger cows than older cows in herds. A decrease in herd size was indicative of a high culling rate. Effective heritability was 0.109. Breeding values ranged from 2.12 for the best cows to -4.80 for the worst cows. This implied that the best and the worst cows were 2.12 times and 4.80 more likely to be culled from herds than the average cow respectively. Genetic trends for functional longevity indicated a marginal decline in sire estimated breeding values. This corresponded with the phenotypic decline in the survivor function observed in cows that calved in 200, 2004 and 2008. All udder, rump, body, feet and leg type traits, with the exception of rear leg side, significantly influenced functional longevity. Farmers culled cows mainly of extremely poor type with a tendency to retain animals with poor to very good structure. Udder traits contributed the most to the risk of a cow being culled. Cows were more likely to be culled for poor scores received for rear teat placement, rear udder height and fore udder attachment, than for any of the other conformation type traits. The contribution of angularity to the risk of being culled was also high, indicating a preference for highly angular bodied cows. Conformation type traits can therefore be used as indicator traits for functional longevity in Holstein cattle, especially when selection and culling is done based on very poor scores for udder traits. The study indicated that genetic improvement in functional longevity can be achieved with the evaluation and selection of dairy sires. There is therefore a wide scope for including functional longevity in the selection objectives for South African Holstein cattle. There is a need to develop appropriate models to ensure that the national dairy industry can benefit from using a Weibull piecewise model.
AFRIKAANSE OPSOMMING: Die verbetering en seleksie van voortreflike diere vir langlewendheid is 'n lewensvatbare en aanvullende strategie vir die verhoging van die funksionele langlewendheid van Holstein melkbeeste. Die genetiese evaluasie van melkproduserende diere ten opsigte van hierdie eienskap is ŉ voorvereiste om verbetering in die eienskap moontlik te maak. Die studie is uitgevoer om nie-genetiese faktore wat die funksionele langlewendheid van Suid-Afrikaanse Holsteins beïnvloed te bepaal, om genetiese parameters vir langlewendheid en vaar teeltwaardes vir dié eienskap te bepaal en ook om die bydrae van bouvorm eienskappe tot die waarskynlikheid van koeie om uitgeskot te word, te bepaal. ŉ Databasis bestaande uit rekords van 166 222 dogters van 2 051 vaars, wat in ses streke in Suid-Afrika (Wes-Kaap, Oos-Kaap, Noord-Kaap, Vrystaat, KwaZulu-Natal en die gekombineerde Gauteng en Noord-Wes Provinsies) voorgekom het, is vir die studie gebruik. Data is ontleed met behulp van 'n stuksgewyse Weibull vader-moeder grootvader model, waarin die basislyn risiko aanvaar is om te verander op 270, 380 dae na kalwing en by afdroog van die melkkoeie. Die vaste effek model het die tyd-onafhanklike effek van ouderdom met eerste kalwing en die tyd-afhanklike effekte van streek en jaar van kalwing, aantal laktasies en binne-kudde melkproduksie klas, binne-kudde proteïen en vet persent produksie klasse volgens jaar van kalwing, asook die gekombineerde tyd-afhanklike effek van kuddegrootte en jaarlikse verandering in kudde grootte, ingesluit. Die ewekansige trop-jaar en vaar effekte is later opgeneem in die model vir bepaling van die vaar- en kudde-jaar variansie komponente. Ontledings het aangedui dat daar 'n algemene afname in die oorlewingsfunksie, wat ŉ aanduiding van funksionele langlewendheid is, was in koeie wat vir die eerste keer in 2000, 2004 en 2008 gekalf het. Koeie wat vir die eerste keer in 2000 gekalf het, het ŉ hoër waarde vir die oorlewingsfunksie gehad as koeie wat onderskeidelik in 2004 en 2008 gekalf het. Alle vaste effekte het betekenisvol tot die waarskynlikheid van ŉ koei om uitgeskot te word, bygedra. Binne-kudde melkproduksie het die grootste bydrae tot die waarskynlikheid van ŉ koei om uitgeskot te word, gemaak. Wanneer die totale laktasieperiode in ag geneem is, was daar gevind dat koeie wat in die tweede fase van laktasie (271-380 dae) die hoogste risiko ervaar het om uitgeskot te word. Hoë produseerders, wanneer hulle met lae produseerders vergelyk is, was minder geneig om uitgeskot te word. Koeie was meer geneig om uitgeskot te word vir lae proteïen produksie persentasie as binne-kudde vet produksie persentasie. Die waarskynlikheid om uitgeskot te word, was laag vir verse wat op ŉ ouderdom van 20 tot 25 maande gekalf het. Die hoër risiko van uitskot van koeie met veelvuldige laktasies het aangedui dat melkboere geneig was om meer jonger koeie te behou en ouer koeie uit te skot. ŉ Afname in kuddegrootte was aanduidend van ŉ hoë uitskotpersentasie. Die effektiewe oorerflikheid was 0.109, met teelwaardes wat van 2.12 vir die hoogste produserende koeie tot -4.80 vir die laagste produserende koeie, gewissel het. Hierdie waardes het dus aangedui dat hoë produseerders en swak produseerders onderskeidelik ŉ 2.12 en 4.80 groter kans gehad het om uitgeskot te kan word. Genetiese tendense vir funksionele langlewendheid het aangedui dat daar 'n marginale afname in vaar beraamde teelwaardes vir funksionele langlewendheid was. Dit het ooreengestem met die waargenome fenotipiese afname in die oorlewingsfunksie wat in 2000, 2004 en 2008 gekalf het. Alle bou-tipe eienskappe (uier, agterkwart, liggaam, voet en been), met die uitsondering van die agterbeen kant, het funksionele langlewendheid betekenisvol beïnvloed. Boere het koeie hoofsaaklik op uiters swak boutipe uitgeskot, met 'n neiging om diere wat as ŉ swak tot uiters goeie bouvorm geklassifiseer is, te behou. Uier eienskappe het die meeste tot die waarskynlikheid van ŉ koei om uitgeskot te word, bygedra. Koeie met ŉ lae gradering vir agterste speen plasing, agter-uier hoogte en voor-uier aanhegting se waarskynlikheid om uitgeskot te word, was groter. Die bydrae van hoekigheid tot die waarskynlikheid van ŉ koei om uitgeskot te word was hoog, wat ŉ aanduiding van ŉ voorkeur vir 'n baie growwe hoekigheid was. Bouvorm tipe eienskappe kan dus gebruik word as indikator eienskappe vir funksionele langlewendheid in Holstein melkbeeste, veral wanneer in ag geneem word dat die meeste melkboere seleksie op grond van swak gradering van uier eienskappe doen. Die studie het aangedui dat genetiese verbetering in funksionele langlewendheid teweeg gebring kan word met die evaluering en seleksie van Holstein vaars. Daar is dus ŉ geleentheid vir die insluiting van funksionele langlewendheid in die seleksie doelwitte vir Suid-Afrikaanse Holstein beeste. Daar is ŉ behoefte om toepaslike modelle te ontwikkel wat met die toepassing daarvan, sal verseker dat die plaaslike melkbedryf sal kan voordeel trek deur die gebruik van 'n stuksgewyse Weibull model benadering.

APA, Harvard, Vancouver, ISO, and other styles

16

Mlotshwa, Vintia Philile. "Modelling hepatotoxicity in HIV/TB co-infected patients: extensions of the Cox Proportional Hazards Model." Master's thesis, Faculty of Science, 2020. http://hdl.handle.net/11427/32806.

Full text

Abstract:

Hepatotoxicity which is also known as liver damage is mainly caused by intake of medicine. It is common among patients who are co-administering Tuberculosis (TB) treatment and the antiretroviral therapy (ART) for the Human Immunodeficiency Viruses (HIV). If severe, hepatotoxicity sometimes necessitates cessation or interruption of treatment. Therefore, understanding, monitoring and managing hepatotoxicity in patients co-infected with TB and HIV is crucial for optimal treatment outcomes. Hepatotoxicity has been investigated in patients coinfected with TB and HIV, however, most studies have analyzed only the first occurrence of hepatotoxicity and discarded information relating to the resolution and recurrence of hepatotoxicity. Data from the ‘Starting Antiretroviral therapy at three Points in Tuberculosis' (SAPiT) trial is used in this project. This was a trial that was instrumental in finalizing treatment guidelines for patients co-infected with HIV and TB in South Africa. The clinical objectives of this project are to estimate incidence rates and determine risk factors associated with hepatotoxicity. The statistical objectives are to fit a Cox regression model, the resolution model of hepatotoxicity, and the extended Cox models for recurring events, including the Andersen Gill (AG) model, the Shared frailty model, the Prentice, Williams and Peterson (PWP) total time (TT) model, the PWP gap time (GT) model, as well as a Cox based recurrent model, that models only the second occurrence of hepatotoxicity. There were 593 patients assessed for hepatotoxicity in the study, 30% (179/593) developed the first occurrence of hepatotoxicity (grade >=1) and 2% (13/593) developed severe hepatotoxicity (grade >=3). Resolved cases (grade = 0) are 76% (136/179) and recurring cases (grade >=1) 24% (32/136). In the Cox multivariable analyses: time-varying treatment arm, older patients, alcohol consumption, low baseline total bilirubin and a positive baseline Hepatitis B surface antigen status, were associated with a higher risk of developing the first occurrence of hepatotoxicity. The extended Cox models (AG model, Shared frailty model, PWP TT model and PWP GT model) in combination identified that: time-varying treatment arm, older patients, alcohol consumption, baseline CD4 count that is greater than 50 cells per mm3 , low baseline total bilirubin, and a positive baseline Hepatitis B surface antigen status were associated with an increased risk of developing recurrent hepatotoxicity. In the resolution model multivariable analyses; non-consumers of alcohol and an abnormal liver function tests at baseline, were associated with an increased chance of resolving the first occurrence of hepatotoxicity. In the multivariable analyses of the recurrent model: younger patients and the time-varying treatment arm were associated with the development of the second occurrence hepatotoxicity. Since the Cox regression model utilized data up to the first occurrence of hepatotoxicity, in some instances, the time-varying treatment effect based on the Cox regression model was closer to unity and marginally significant. And the corresponding effect based on the recurrent event models (AG model, Shared frailty model, PWP TT model, PWP GT model and the recurrent model), that utilized data of the first and second occurrence of hepatotoxicity, generally produced a time-varying treatment effect slightly far from unity with a strong statistical significance. This trend was similar for other predictors of hepatotoxicity, like CD4 count and alcohol consumption. In conclusion, hepatotoxicity is common in this study, however, it is often transient or mild and did not necessitate treatment interruption. However, close monitoring of patients especially in the first 5 months of TB-treatment is recommended. The PWP TT model seemed to be the best model for modelling recurring hepatotoxicity, since the identified risk factors that were associated with hepatotoxicity, changed from the first occurrence of hepatotoxicity to the second occurrence of hepatotoxicity.

APA, Harvard, Vancouver, ISO, and other styles

17

Söderberg, Daniel. "Model estimation of the longevity for cars registered in Sweden using survival analysis and Cox proportional hazards model." Thesis, Uppsala universitet, Statistiska institutionen, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-227520.

Full text

Abstract:

Time-to-event data is used in this thesis to analyze private cars’ longevity in Sweden. Thedataset is provided by Trafikanalys and contains all registered, deregistered or temporary deregisteredcars in Sweden during the time period 2000 - 2012.A Cox proportional hazards model is fitted, including variables such as car manufacturer andcar body. The results show that directly imported cars have a much shorter median survivalcompared to non-imported cars. The convertible cars have the longest median survival amongthe five different car bodies. Sedan and station wagon body types have the shortest mediansurvival. Volvo and Mercedes have the longest survival while Renault, Ford and Opel have theshortest survival. The model fits the data reasonably well, and the assumption of proportionalhazards holds for most of the variables.

APA, Harvard, Vancouver, ISO, and other styles

18

Frejd, Ellen, and Jenny Sjödin. "Är kognitiva test relaterade till demens? : En utvidgning av Cox Proportional Hazards Model med tidsvarierande kovariat." Thesis, Umeå universitet, Statistik, 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-184984.

Full text

Abstract:

Syftet med uppsatsen är att undersöka samband mellan kognitiva test och insjuknande i demens. Demens är en samlingsdiagnos för sjukdomar som är kopplade till nedsatt kognitiv förmåga. Symptom inkluderar försämrad minnesfunktion och personlighetsförändringar. Data är insamlad av den longitudinella studien Betula vid Umeå universitet. Vid upprepade testtillfällen genomfördes minnestester för att mäta deltagarnas minnesfunktion och kognitiva förmåga. Minnestestresultat analyseras med en utvidgning av Cox Proportional Hazards Model med tidsvarierande kovariat. Genom att analysera testresultat som varierar över tid erhölls uppdaterad information om deltagarnas kognitiva tillstånd. Vidare jämförs den utvidgade modellen med en klassisk Coxmodell med baslinjedata. Modellering inkluderar även kontrollvariabeln ApoE4 som är en genvariant som innebär förhöjd demensrisk. Resultaten visar samband mellan demens och test som mäter episodminne och visuospatial förmåga. Den utvidgade modellen med tidsvarierande testresultat föredras som modellval.

APA, Harvard, Vancouver, ISO, and other styles

19

Bertke, Stephen J. "A Simulation Study of the Cox Proportional Hazards Model and the Nested Case-Control Study Design." University of Cincinnati / OhioLINK, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1307321495.

Full text

APA, Harvard, Vancouver, ISO, and other styles

20

Zhao, Yonggang. "The general linear model for censored data." The Ohio State University, 2003. http://rave.ohiolink.edu/etdc/view?acc_num=osu1054781042.

Full text

APA, Harvard, Vancouver, ISO, and other styles

21

Demissie, Mathewos S. "Investigating center effects in a multi-center clinical trial study using a parametric proportional hazards meta-analysis model." College Park, Md.: University of Maryland, 2009. http://hdl.handle.net/1903/9588.

Full text

Abstract:

Thesis (M.S.) -- University of Maryland, College Park, 2009.
Thesis research directed by: Mathematical Statistics Program. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

APA, Harvard, Vancouver, ISO, and other styles

22

Charoensawat, Supada. "A likelihood approach based upon the proportional hazards model for SROC modelling in meta-analysis of diagnostic studies." Thesis, University of Reading, 2014. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.627926.

Full text

Abstract:

The number of meta-analysis of diagnostic studies is increasing and the models which deal with the summary receiver operating characteristic (SROC) have become quite popular. Many of these models have reached considerable statistical complexity, required expertise and knowledge. Here" a model named the proportional hazard model (PHM) is developed. The PHM model has a simple form and is easy to interpret. There is only one parameter of interest 0, which is called the diagnostic accuracy and has the interpretation that the smaller 0 is, the higher the diagnostic accuracy.

APA, Harvard, Vancouver, ISO, and other styles

23

Ekman, Anna. "Variable selection for the Cox proportional hazards model : A simulation study comparing the stepwise, lasso and bootstrap approach." Thesis, Umeå universitet, Institutionen för matematik och matematisk statistik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-130521.

Full text

Abstract:

In a regression setting with a number of measured covariates not all may be relevant to the response. By reducing the numbers of covariates included in the final model we could improve its prediction accurarcy as well as making it easier to interpret. In survival analysis, the study of time-to-event data, the most common form of regression is the semi-parametric Cox proportional hazard (PH) model. In this thesis we have compared three different ways to perform variable selection in the Cox PH model, stepwise regression, lasso and bootstrap. By simulating survival data we could control which covariates that were significant for the response. Fitting the Cox PH model to these data using the three different variable selection methods we could evaluate how well each method performs in finding the correct model. We found that while bootstrap in some cases could improve the stepwise approach its performance is strongly effected by the choice of inclusion frequency. Lasso performed equivalent or slightly better than the stepwise method for data with weak effects. However, when the data instead consists of strong effects, the performance of stepwise is considerably better than the performance of lasso.
Vid regression söks sambandet mellan en beroende variabel och en eller flera förklarande variabler. Även om vi har tillgång till många förklarande variabler är det dock inte säkert att alla påverkar den beroende variabeln. Genom att minska antalet variabler som inkluderas i den slutgiltiga modellen kan man förbättra dess prediktionsförmåga samtidigt som den blir lättare att tolka. Inom överlevnadslys är en av de vanligaste regressionsmetoderna den semi-parametriska Cox proportional hazard (PH) model. I den här uppsatsen har vi jämfört tre olika metoder för variabel selektion i Cox PH model, stegvis regression, lasso och bootstrap. Genom att simulera överlevnadsdata kan vi styra vilka variabler som påverkar den beroende variabelen. Det blir då möjligt att utvärdera hur väl de olika metoderna lyckas med att inkludera dessa variabler i den slutgiltiga Cox PH model. Vi fann att bootstrap i vissa situationer gav bättre resultat än den stegvisa regressionen, dock varierar resultatet väldigt mycket beroende på valet av inklusionsfrekvens. Resultaten av lasso och stegvis regression är likvärdiga, eller till fördel för lasso, så länge datat innehåller svagare effekter. När datat istället består av starkare effekter ger dock den stegvisa regressionen mycket bättre resultat än lasso.

APA, Harvard, Vancouver, ISO, and other styles

24

Carstens, Wiehahn Alwyn. "Regression analysis of caterpillar 793D haul truck engine failure data and through-life diagnostic information using the proportional hazards model." Thesis, Stellenbosch : Stellenbosch University, 2012. http://hdl.handle.net/10019.1/20333.

Full text

Abstract:

Thesis (MScEng)--Stellenbosch University, 2012.
ENGLISH ABSTRACT: Physical Asset Management (PAM) is becoming a greater concern for companies in industry today. The widely accepted British Standards Institutes’ specification for optimized management of physical assets and infrastructure is PAS55. According to PAS55, PAM is the “systematic and co-ordinated activities and practices through which an organization optimally manages its physical assets, and their associated performance, risks and expenditures over their life cycle for the purpose of achieving its organizational strategic plan”. One key performance area of PAM is Asset Care Plans (ACP). These plans are maintenance strategies which improve or ensure acceptable asset reliability and performance during its useful life. Maintenance strategies such as Condition Based Maintenance (CBM) acts upon Condition Monitoring (CM) data, disregarding the previous failure histories of an asset. Other maintenance strategies, such as Usage Based Maintenance (UBM), is based on previous failure histories, and does not consider CM data. Regression models make use of both CM data and previous failure histories to develop a model which represents the underlying failure behaviour of the asset under study. These models can be of high value in ACP development due to the fact that Residual Useful Life (RUL) can be estimated and/or the long term life cycle cost can be optimized. The objective of this thesis was to model historical failure data and CM data well enough so that RUL or optimized preventive maintenance instant estimations can be made. These estimates were used in decision models to develop maintenance schedules, i.e. ACPs. Several regression models were evaluated to determine the most suitable model to achieve the objectives of this thesis. The model found to be most suitable for this research project was the Proportional Hazards Model (PHM). A comprehensive investigation on the PHM was undertaken focussing on the mathematics and the practical implementation thereof. Data obtained from the South African mining industry was modelled with the Weibull PHM. It was found that the developed model produced estimates which were accurate representations of reality. These findings provide an exciting basis for the development of futureWeibull PHMs that could result in huge maintenance cost savings and reduced failure occurrences.
AFRIKAANSE OPSOMMING: Fisiese Bate Bestuur (FBB) is besig om ’n groter bekommernis vir maatskappye in die bedryf te word. Die Britse Standaarde Instituut se spesifikasie vir optimale bestuur van fisiese bates en infrastruktuur is PAS55. Volgens PAS55 is FBB die “sistematiese en gekoördineerde aktiwiteite en praktyke wat deur ’n organisasie optimaal sy fisiese bates, hul verwante prestasie, risiko’s en uitgawes vir die doel van die bereiking van sy organisatoriese strategiese plan beheer oor hul volle lewensiklus te bestuur”. Een Sleutel Fokus Area (SFA) van FBB is Bate Versorgings Plan (BVP) ontwikkeling. Hierdie is onderhouds strategieë wat bate betroubaarheid verbeter of verseker tydens die volle bruikbare lewe van die bate. Een onderhoud strategie is Toestands Gebasseeerde Onderhoud (TGO) wat besluite baseer op Toestand Monitering (TM) informasie maar neem nie die vorige falingsgeskiedenis van die bate in ag nie. Ander onderhoud strategieë soos Gebruik Gebasseerde Onderhoud (GGO) is gebaseer op historiese falingsdata maar neem nie TM inligting in ag nie. Regressiemodelle neem beide TM data en historiese falings geskiedenis data in ag ten einde die onderliggende falings gedrag van die gegewe bate te verteenwoordig. Hierdie modelle kan baie nuttig wees vir BVP ontwikkeling te danke aan die feit dat Bruikbare Oorblywende Lewe (BOL) geskat kan word en/of die langtermyn lewenssilus koste geoptimeer kan word. Die doelwit van hierdie tesis was om historiese falingsdata en TT data goed genoeg te modelleer sodat BOL of optimale langtermyn lewensiklus kostes bepaal kan word om opgeneem te word in BVP ontwikkeling. Hierdie bepalings word dan gebruik in besluitnemings modelle wat gebruik kan word om onderhoud skedules op te stel, d.w.s. om ’n BVP te ontwikkel. Verskeie regressiemodelle was geëvalueer om die regte model te vind waarmee die doel van hierdie tesis te bereik kan word. Die mees geskikte model vir die navorsingsprojek was die Proporsionele Gevaarkoers Model (PGM). ’n Omvattende ondersoek oor die PGM is onderneem wat fokus op die wiskunde en die praktiese implementering daarvan. Data is van die Suid-Afrikaanse mynbedryf verkry en is gemodelleer met behulp van die Weibull PGM. Dit was bevind dat die ontwikkelde model resultate geproduseer het wat ’n akkurate verteenwoordinging van realiteit is. Hierdie bevindinge bied ’n opwindende basis vir die ontwikkeling van toekomstige Weibull Proporsionele Gevaarkoers Modelle wat kan lei tot groot onderhoudskoste besparings en minder onverwagte falings.

APA, Harvard, Vancouver, ISO, and other styles

25

Boudreau, Christian. "Duration Data Analysis in Longitudinal Survey." Thesis, University of Waterloo, 2003. http://hdl.handle.net/10012/1043.

Full text

Abstract:

Considerable amounts of event history data are collected through longitudinal surveys. These surveys have many particularities or features that are the results of the dynamic nature of the population under study and of the fact that data collected through longitudinal surveys involve the use of complex survey designs, with clustering and stratification. These particularities include: attrition, seam-effect, censoring, left-truncation and complications in the variance estimation due to the use of complex survey designs. This thesis focuses on the last two points. Statistical methods based on the stratified Cox proportional hazards model that account for intra-cluster dependence, when the sampling design is uninformative, are proposed. This is achieved using the theory of estimating equations in conjunction with empirical process theory. Issues concerning analytic inference from survey data and the use of weighted versus unweighted procedures are also discussed. The proposed methodology is applied to data from the U. S. Survey of Income and Program Participation (SIPP) and data from the Canadian Survey of Labour and Income Dynamics (SLID). Finally, different statistical methods for handling left-truncated sojourns are explored and compared. These include the conditional partial likelihood and other methods, based on the Exponential or the Weibull distributions.

APA, Harvard, Vancouver, ISO, and other styles

26

Ansin, Elin. "An evaluation of the Cox-Snell residuals." Thesis, Uppsala universitet, Statistiska institutionen, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-256665.

Full text

Abstract:

It is common practice to use Cox-Snell residuals to check for overall goodness of tin survival models. We evaluate the presumed relation of unit exponentially dis-tributed residuals for a good model t and evaluate under some violations of themodel. This is done graphically with the usual graphs of Cox-Snell residual andformally using Kolmogorov-Smirnov goodness of t test. It is observed that residu-als from a correctly tted model follow unit exponential distribution. However, theCox-Snell residuals do not seem to be sensitive to the violations of the model.

APA, Harvard, Vancouver, ISO, and other styles

27

Wang, Xiaohui. "Bayesian classification and survival analysis with curve predictors." [College Station, Tex. : Texas A&M University, 2006. http://hdl.handle.net/1969.1/ETD-TAMU-1205.

Full text

APA, Harvard, Vancouver, ISO, and other styles

28

Youssef, Ibrahim Mohamed. "Multi-Platform Molecular Data Integration and Disease Outcome Analysis." Diss., Virginia Tech, 2016. http://hdl.handle.net/10919/73580.

Full text

Abstract:

One of the most common measures of clinical outcomes is the survival time. Accurately linking cancer molecular profiling with survival outcome advances clinical management of cancer. However, existing survival analysis relies intensively on statistical evidence from a single level of data, without paying much attention to the integration of interacting multi-level data and the underlying biology. Advances in genomic techniques provide unprecedented power of characterizing the cancer tissue in a more complete manner than before, opening the opportunity of designing biologically informed and integrative approaches for survival analysis. Many cancer tissues have been profiled for gene expression levels and genomic variants (such as copy number alterations, sequence mutations, DNA methylation, and histone modification). However, it is not clear how to integrate the gene expression and genetic variants to achieve a better prediction and understanding of the cancer survival. To address this challenge, we propose two approaches for data integration in order to both biologically and statistically boost the features selection process for proper detection of the true predictive players of survival. The first approach is data-driven yet biologically informed. Consistent with the biological hierarchy from DNA to RNA, we prioritize each survival-relevant feature with two separate scores, predictive and mechanistic. With mRNA expression levels in concern, predictive features are those mRNAs whose variation in expression levels are associated with the survival outcome, and mechanistic features are those mRNAs whose variation in expression levels are associated with genomic variants (copy number alterations (CNAs) in this study). Further, we propose simultaneously integrating information from both the predictive model and the mechanistic model through our new approach GEMPS (Gene Expression as a Mediator for Predicting Survival). Applied on two cancer types (ovarian and glioblastoma multiforme), our method achieved better prediction power than peer methods. Gene set enrichment analysis confirms that the genes utilized for the final survival analysis are biologically important and relevant. The second approach is a generic mathematical framework to biologically regularize the Cox's proportional hazards model that is widely used in survival analysis. We propose a penalty function that both links the mechanistic model to the clinical model and reflects the biological downstream regulatory effect of the genomic variants on the mRNA expression levels of the target genes. Fast and efficient optimization principles like the coordinate descent and majorization-minimization are adopted in the inference process of the coefficients of the Cox model predictors. Through this model, we develop the regulator-target gene relationship to a new one: regulator-target-outcome relationship of a disease. Assessed via a simulation study and analysis of two real cancer data sets, the proposed method showed better performance in terms of selecting the true predictors and achieving better survival prediction. The proposed method gives insightful and meaningful interpretability to the selected model due to the biological linking of the mechanistic model and the clinical model. Other important forms of clinical outcomes are monitoring angiogenesis (formation of new blood vessels necessary for tumor to nourish itself and sustain its existence) and assessing therapeutic response. This can be done through dynamic imaging, in which a series of images at different time instances are acquired for a specific tumor site after injection of a contrast agent. Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) is a noninvasive tool to examine tumor vasculature patterns based on accumulation and washout of the contrast agent. DCE-MRI gives indication about tumor vasculature permeability, which in turn indicates the tumor angiogenic activity. Observing this activity over time can reflect the tumor drug responsiveness and efficacy of the treatment plan. However, due to the limited resolution of the imaging scanners, a partial-volume effect (PVE) problem occurs, which is the result of signals from two or more tissues combining together to produce a single image concentration value within a pixel, with the effect of inaccurate estimation to the values of the pharmacokinetic parameters. A multi-tissue compartmental modeling (CM) technique supported by convex analysis of mixtures is used to mitigate the PVE by clustering pixels and constructing a simplex whose vertices are of a single compartment type. CAM uses the identified pure-volume pixels to estimate the kinetics of the tissues under investigation. We propose an enhanced version of CAM-CM to identify pure-volume pixels more accurately. This includes the consideration of the neighborhood effect on each pixel and the use of a barycentric coordinate system to identify more pure-volume pixels and to test those identified by CAM-CM. Tested on simulated DCE-MRI data, the enhanced CAM-CM achieved better performance in terms of accuracy and reproducibility.
Ph. D.

APA, Harvard, Vancouver, ISO, and other styles

29

Brännmark, My, and Ellen Fors. "Modellering av åtgärdsintervall för vägar med tung trafik." Thesis, Umeå universitet, Institutionen för matematik och matematisk statistik, 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-160057.

Full text

Abstract:

In Sweden, there has been an long term effort to allow as heavy traffic as possible, provided thatthe road network can handle it. This is because heavy traffic offers a competitive advantage withsocio-economic gains. In July 2018, the Swedish Transport Administration made 12 percent ofthe Swedish road network avaliable for the new maximum vehicle weight of 74 tonnes, basedon a legislative change from 2017. It is known that heavy traffic has a negative effect on thedegradation of the road, but it prevails divided opinions on whether 74 tonnes have a greaterimpact on the degradation rate compared to previous maximum gross weights of 64 tonnes.The 74 tonne vehicles have the same allowed axle load, which means more axles per vehicle. Some argue that an increased total load and more axles affect the degradation associated withtime-dependent material properties, while others argue that 74 tonnes mean fewer heavy vehiclesoverall, and thus should have a positive impact on the road’s lifespan. The construction companySkanska therefore requests a statistical analysis that enables to nuance the effects that heavytraffic has on the Swedish state road network. Since there is very limited data on the effect of 74 tonne traffic, this Master thesis instead focuseson modeling heavy traffic in general in order to be able to draw conclusions on which variablesare significant for a road’s lifetime. The method used is survival analysis where the lifetimeof the road is defined as the time between two maintenance treatments. The model selectedis the semi-parametric ’Cox Proportional Hazard Model’. The model is fitted with data froman open source database called LTPP (Long Term Pavement Performance) which is providedby the National Road and Transport Research Institute (VTI). The result of the modeling ispresented with hazard ratios, which is the relative risk that a road will require maintance atthe next time stamp compared to a reference category. The covariates that turned out to besignificant for a road’s lifetime and thus are included in the model are; lane width, undergroundtype, speed limit, asphalt layer thickness, bearing layer thickness and proportion of heavy traffic. Survival curves estimated by the model are also presented. In addition, a sensitivity analysis ismade by exploring survival curves estimated for different scenarios, with different combinationsof covariate levels.The results is then compared with previous studies on the subject. The most interesting finding isa case study from Finland since Finland allow 76 tonne vehicles since 2013. In the comparison,the model’s significant variables are confirmed, but the significance of precipitation and thenumber of axes for a roads lifetime is also highlighted

APA, Harvard, Vancouver, ISO, and other styles

30

Hallström, Richard. "Estimating Loss-Given-Default through Survival Analysis : A quantitative study of Nordea's default portfolio consisting of corporate customers." Thesis, Umeå universitet, Institutionen för matematik och matematisk statistik, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-122914.

Full text

Abstract:

In Sweden, all banks must report their regulatory capital in their reports to the market and their models for calculating this capital must be approved by the financial authority, Finansinspektionen. The regulatory capital is the capital that a bank has to hold as a security for credit risk and this capital should serve as a buffer if they would loose unexpected amounts of money in their lending business. Loss-Given-Default (LGD) is one of the main drivers of the regulatory capital and the minimum required capital is highly sensitive to the reported LGD. Workout LGD is based on the discounted future cash flows obtained from defaulted customers. The main issue with workout LGD is the incomplete workouts, which in turn results in two problems for banks when they calculate their workout LGD. A bank either has to wait for the workout period to end, in which some cases take several years, or to exclude or make rough assumptions about those incomplete workouts in their calculations. In this study the idea from Survival analysis (SA) methods has been used to solve these problems. The mostly used SA model, the Cox proportional hazards model (Cox model), has been applied to investigate the effect of covariates on the length of survival for a monetary unit. The considered covariates are Country of booking, Secured/Unsecured, Collateral code, Loan-To-Value, Industry code, Exposure-At- Default and Multi-collateral. The data sample was first split into 80 % training sample and 20 % test sample. The applied Cox model was based on the training sample and then validated with the test sample through interpretation of the Kaplan-Meier survival curves for risk groups created from the prognostic index (PI). The results show that the model correctly rank the expected LGD for new customers but is not always able to distinguish the difference between risk groups. With the results presented in the study, Nordea can get an expected LGD for newly defaulted customers, given the customers’ information on the considered covariates in this study. They can also get a clear picture of what factors that drive a low respectively high LGD.
I Sverige måste alla banker rapportera sitt lagstadgade kapital i deras rapporter till marknaden och modellerna för att beräkna detta kapital måste vara godkända av den finansiella myndigheten, Finansinspektionen. Det lagstadgade kapitalet är det kapital som en bank måste hålla som en säkerhet för kreditrisk och den agerar som en buffert om banken skulle förlora oväntade summor pengar i deras utlåningsverksamhet. Loss- Given-Default (LGD) är en av de främsta faktorerna i det lagstadgade kapitalet och kravet på det minimala kapitalet är mycket känsligt för det rapporterade LGD. Workout LGD är baserat på diskonteringen av framtida kassaflöden från kunder som gått i default. Det huvudsakliga problemet med workout LGD är ofullständiga workouts, vilket i sin tur resulterar i två problem för banker när de ska beräkna workout LGD. Banken måste antingen vänta på att workout-perioden ska ta slut, vilket i vissa fall kan ta upp till flera år, eller så får banken exkludera eller göra grova antaganden om dessa ofullständiga workouts i sina beräkningar. I den här studien har idén från Survival analysis (SA) metoder använts för att lösa dessa problem. Den mest använda SA modellen, Cox proportional hazards model (Cox model), har applicerats för att undersöka effekten av kovariat på livslängden hos en monetär enhet. De undersökta kovariaten var Land, Säkrat/Osäkrat, Kollateral-kod, Loan-To-Value, Industri-kod Exposure-At-Default och Multipla-kollateral. Dataurvalet uppdelades först i 80 % träningsurval och 20 % testurval. Den applicerade Cox modellen baserades på träningsurvalet och validerades på testurvalet genom tolkning av Kaplan-Meier överlevnadskurvor för riskgrupperna skapade från prognosindexet (PI). Med de presenterade resultaten kan Nordea beräkna ett förväntat LGD för nya kunder i default, givet informationen i den här studiens undersökta kovariat. Nordea kan också få en klar bild över vilka faktorer som driver ett lågt respektive högt LGD.

APA, Harvard, Vancouver, ISO, and other styles

31

Latorre, Maria do Rosario Dias de Oliveira. "Comparação entre alguns métodos estatísticos em análise de sobrevivência: aplicação em uma coorte de pacientes com câncer de pênis." Universidade de São Paulo, 1996. http://www.teses.usp.br/teses/disponiveis/6/6132/tde-12112014-153823/.

Full text

Abstract:

O objetivo deste trabalho foi comparar o desempenho do modelo de riscos proporcionais de Cox convencional, modelo de Cox modificado quando os riscos não são proporcionais e o modelo de análise de sobrevida baseado na teoria de processos de contagem. Para tanto utilizou-se uma coorte de 648 pacientes portadores de câncer de pênis, atendidos no Departamento de Cirurgia Pélvica do Hospital A. C. Camargo, no período de 1953 a 1985. Dessa coorte foram selecionadas três amostras com o objetivo de validar internamente os resultados da análise de sobrevida do banco de dados original. Os resultados do modelo de riscos proporcionais de Cox, no banco de dados original, foram confirmados por uma das amostras desse conjunto de dados. Apenas o estadiamento N foi confirmado como fator prognóstico também nas outras duas amostras. O modelo de riscos proporcionais de Cox e o modelo de análise de sobrevida baseado na teoria de processos de contagem apresentaram resultados semelhantes, na definição dos fatores prognósticos dessa coorte de pacientes com câncer de pênis. O modelo utilizando processos de contagem é mais sofisticado, do ponto de vista matemático. Porém o modelo de Cox está disponível em grande número de pacotes estatísticos e a interpretação de seus coeficientes se faz com maior facilidade. Por isso, talvez, continue a ser a técnica estatística mais utilizada quando o objetivo do estudo é definir fatores prognósticos e grupos de risco. Os fatores prognósticos para a sobrevida de pacientes com câncer de pênis foram os estadiamentos T e N e o grau de diferenciação do tumor. Esses resultados foram ajustados pelo ano de início de tratamento no Hospital A.C. Camargo. Os pacientes com prognóstico favorável foram os que apresentaram tumor pequeno, sem presença de linfonodos clinicamente positivos, e tumor bem diferenciado.
The aim of this study was to compare the performance of the Cox proportional hazards model, the Cox model with time-dependent covariates and the survival model using the counting process theory. These methods were applied in a cohort of 648 patients with penile cancer treated at the Department of Pelvic Surgery, Hospital A.C. Camargo (São Paulo-Brazil), between 1953 and 1985. Three samples were selected from the total database in order to check the internal validity. The prognostic factors selected using the Cox proportional hazards model were the same in one sample. The only prognostic factor selected in all samples was the N stage. The T and N stages, and the grade of differentiation were independent prognostic factors of survival using both the Cox proportional hazards model and the survival,model using the counting process theory. The statistical significance was the same and even the values of estimation of the coefficients were very close. The survival model using the counting process is more sophisticated from the mathematical point of view, but the Cox model is more available in statistical software, and, probably because of this, is more applied in survival analysis than the model using the counting processo Patients with small tumors, clinically negatives nodes and well differentiated tumors showed a favorable prognosis. These results were adjusted by year of the beginning in the study.

APA, Harvard, Vancouver, ISO, and other styles

32

Zhao, Feng. "Bootstrap variable selection and model validation for Cox's proportional hazards regression models, with applications to the identification of factors predictive of overall and post-relapse survival in advanced epithelial ovarian cancer." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk2/tape17/PQDD_0026/MQ31275.pdf.

Full text

APA, Harvard, Vancouver, ISO, and other styles

33

Chauvel, Cecile. "Processus empiriques pour l'inférence dans le modèle de survie à risques non proportionnels." Thesis, Paris 6, 2014. http://www.theses.fr/2014PA066399/document.

Full text

Abstract:

Nous nous intéressons à des processus empiriques particuliers pour l'inférence dans le modèle à risques non proportionnels. Ce modèle permet au coefficient de régression de varier avec le temps et généralise le modèle à risques proportionnels très utilisé pour modéliser des données de survie. Le processus du score standardisé que nous étudions est une somme séquentielle des résidus standardisés du modèle. Le processus est considéré en présence d'une covariable dans le modèle, avant d'être étendu au cas de multiples covariables pouvant être corrélées. Le plan du manuscrit se décompose en trois parties. Dans un premier temps, nous établissons les propriétés limites du processus sous le modèle et sous un modèle mal spécifié. Dans une deuxième partie, nous utilisons les résultats de convergence du processus pour dériver des tests de la valeur du paramètre du modèle. Nous montrons qu'un des tests proposés est asymptotiquement équivalent au test de référence du log-rank pour comparer les fonctions de survie de plusieurs groupes de patients. Nous construisons des tests plus puissants que le test du log-rank sous certaines alternatives. Enfin, dans la dernière partie, nous étudions comment lier prédiction et adéquation dans le modèle à risques non proportionnels. Nous proposons une méthode de construction d'un modèle bien ajusté en maximisant sa capacité prédictive. Aussi, nous introduisons un test d'adéquation du modèle à risques proportionnels. Les performances des méthodes proposées, qu'il s'agisse des tests sur le paramètre ou de l'adéquation du modèle, sont comparées à des méthodes de référence par des simulations. Les méthodes sont illustrées sur des données réelles
In this thesis, we focus on particular empirical processes on which we can base inference in the non-proportional hazards model. This time-varying coefficient model generalizes the widely used proportional hazards model in the field of survival analysis. Our focus is on the standardized score process that is a sequential sum of standardized model-based residuals. We consider first the process with one covariate in the model, before looking at its extension for multiple and possibly correlated covariates. The outline of the manuscript is composed of three parts. In the first part, we establish the limit properties of the process under the model as well as under a misspecified model. In the second part, we use these convergence results to derive tests for the value of the model parameter. We show that one proposed test is asymptotically equivalent to the log-rank test, which is a benchmark for comparing survival experience of two or more groups. We construct more powerful tests under some alternatives. Finally, in the last part, we propose a methodology linking prediction and goodness of fit in order to construct models. The resulting models will have a good fit and will optimize predictive ability. We also introduce a goodness-of-fit test of the proportional hazards model. The performances of our methods, either tests for the parameter value or goodness-of-fit tests, are compared to standard methods via simulations. The methods are illustrated on real life datasets

APA, Harvard, Vancouver, ISO, and other styles

34

Malvestio, Marisa Aparecida Amaro. "Predeterminantes de sobrevivência em vítimas de acidentes de trânsito submetidas a atendimento pré-hospitalar de suporte avançado à vida." Universidade de São Paulo, 2005. http://www.teses.usp.br/teses/disponiveis/7/7139/tde-15012007-165049/.

Full text

Abstract:

O Atendimento Pré Hospitalar (APH) é um importante recurso no atendimento à vítimas de trauma. No entanto, há muitas dificuldades para demonstrar o efeito benéfico das intervenções do APH na sobrevivência das vítimas, sobretudo as de suporte avançado à vida (SAV). A proposta deste estudo é caracterizar as vítimas de acidentes trânsito, com Revised Trauma Score (RTS) <11, atendidas pelo SAV municipal e encaminhadas a hospitais terciários em São Paulo, além de identificar as variáveis da fase pré-hospitalar associadas à sobrevivência e avaliar o valor predeterminante dessas variáveis sobre o resultado obtido pelas vítimas. As variáveis avaliadas foram: sexo, idade, mecanismos do acidente, procedimentos de suporte básico e SAV realizados, repercussão fisiológica do trauma na cena do acidente, (considerando o RTS , seus parâmetros e flutuações), o tempo consumido no APH, gravidade do trauma segundo o Injury Severity Score (ISS),a Maximum Abbreviated Injury Scale (MAIS) e número de lesões para cada segmento corporal. Os resultados obtidos por 175 vítimas entre 12 e 65 anos, foram submetidos a Análise de Sobrevivência de Kaplan Meier e ao Modelo de Riscos Proporcionais de Cox. A variável dependente foi o tempo de sobrevivência após o acidente, considerando os intervalos até 6h,12h, 24h, 48h, até 7 dias e até o término da internação. Os homens (86,9%) e a faixa etária de 20 a 29 anos (36,0%) foram as mais freqüentes. Os atropelamentos (45,1%) e o envolvimento de motocicletas e seus ocupantes (30,9%) foram os destaques dentre os mecanismos de trauma. A média do RTS na cena e do ISS, foram respectivamente 8,8 e 19,4.Os segmentos corpóreos mais atingidos foram: cabeça (58,8%), membros inferiores (45,1%) e superfície externa (40%). A média de tempo consumido na fase de APH foi 41min (tempo de cena 20,2min). Ocorreram 36% de óbitos, (metade em até 6 horas). A análise estatística revelou 24 fatores associados à sobrevivência, dentre eles, os procedimentos respiratórios avançados e os circulatórios básicos, as variáveis relativas ao RTS e a gravidade (ISS, MAIS e o número de lesões). No modelo final de Cox, ter sido submetido a procedimentos respiratórios avançados, compressões torácicas, apresentar lesão abdominal e ISS>25, foi associado a maior risco para o óbito até 48h após o trauma. Até 7 dias, a compressão torácica não se manteve no modelo final e a PAS de zero a 75mmHg apresentou associação com a morte após o acidente. Até a alta hospitalar, a ausência de PAS na avaliação inicial permaneceu no modelo. A reposição de volume foi o único fator com valor protetor para o risco de óbito presente em todos os momentos
The prehospital care (PH) is an important resource to trauma victims care. Nevertheless, there is great difficulty in demonstrating the PH interventions positive effect in victims survival, especially when concerning the advanced life support (ALS). The aim of this study is to characterize motor vehicle crash victims with Revised Trauma Score (RTS) <11 cared by municipal ALS and moved to tertiary hospitals in São Paulo in addition to identifying the prehospital variables associated to survival, and to evaluate their values as victim survival outcome determinant. The variables evaluated were: sex, age, trauma mechanism, basic life support and ALS procedures, physiological measures in the accident scene (considering the RTS, its parameters and fluctuations), the time consumed in PH phase, trauma severity by Injury Severity Score (ISS), the Maximum Abbreviated Injury Scale (MAIS) and number of lesions in each body region. The main results obtained by 175 victims between 12 e 65 years of age were submitted to the Kaplan Meier Survival Analysis and to Cox Proportional hazards Regression Analysis. The dependent variable was the survival time after the motor vehicle accident considering the intervals up to 6,12,24 and 48hs , up to 7 days and until the time of hospital discharge. Men (86,9%) and the 20 to 29 aged group (36%) were the most frequent. The pedestrians struck by car (45,1%) and the motorcycles (and their riders) (30,9%)were the highlight in trauma mechanisms. The RTS and the ISS average were 8,8 and 19,4 respectively. The more damaged body regions were head (58,8%), lower limbs (45,1%) and external surface (40%).The prehospital time average was 41 min (scene time 20,2min).Death rate was 36% (half of which up to 6hs).The statistical analysis revealed 24 survival associated factors. The ALS and the circulatory basic procedures, the RTS variables and the trauma severity (ISS,MAIS and number of lesions) were within them. In the final Cox Model were associated to higher risk of death up to 48hs after trauma: the submission to ALS respiratory procedures, chest compressions, the presence of abdominal injuries and ISS>25 .Until the 7th day the chest compression was not sustained in a final model and the systolic blood pressure (SBP) from zero to 75mmHg revealed statistical association with death after trauma. Until hospital discharge the SBP absence in scene evaluation remained in the model. The prehospital intravenous fluid refilling was the only factor of protector value to death risk in all moments

APA, Harvard, Vancouver, ISO, and other styles

35

Maposa, Innocent. "Survival modelling and analysis of HIV/AIDS patients on HIV care and antiretroviral treatment to determine longevity prognostic factors." University of the Western Cape, 2016. http://hdl.handle.net/11394/5444.

Full text

Abstract:

Philosophiae Doctor - PhD
The HIV/AIDS pandemic has been a torment to the African developmental agenda, especially the Southern African Development Countries (SADC), for the past two decades. The disease and condition tends to affect the productive age groups. Children have also not been spared from the severe effects associated with the disease. The advent of antiretroviral treatment (ART) has brought a great relief to governments and patients in these regions. More people living with HIV/AIDS have experienced a boost in their survival prospects and hence their contribution to national developmental projects. Survival analysis methods are usually used in biostatistics, epidemiological modelling and clinical research to model time to event data. The most interesting aspect of this analysis comes when survival models are used to determine risk factors for the survival of patients undergoing some treatment or living with a certain disease condition. The purpose of this thesis was to determine prognostic risk factors for patients' survival whilst on ART. The study sought to highlight the risk factors that impact the survival time negatively at different survival time points. The study utilized a sample of paediatric and adult datasets from Namibia and Zimbabwe respectively. The paediatric dataset from Katutura hospital (Namibia) comprised of the adolescents and children on ART, whilst the adult dataset from Bulawayo hospital (Zimbabwe) comprised of those patients on ART in the 15 years and above age categories. All datasets used in this thesis were based on retrospective cohorts followed for some period of time. Different methods to reduce errors in parameter estimation were employed to the datasets. The proportional hazards, Bayesian proportional hazards and the censored quantile regression models were utilized in this study. The results from the proportional hazards model show that most of the variables considered were not signifcant overall. The Bayesian proportional hazards model shows us that all the considered factors had different risk profiles at the different quartiles of the survival times. This highlights that by using the proportional hazards models, we only get a fixed constant effect of the risk factors, yet in reality, the effect of risk factors differs at different survival time points. This picture was strongly highlighted by the censored quantile regression model which indicated that some variables were significant in the early periods of initiation whilst they did not significantly affect survival time at any other points in the survival time distribution. The censored quantile regression models clearly demonstrate that there are significant insights gained on the dynamics of how different prognostic risk factors affect patient survival time across the survival time distribution compared to when we use proportional hazards and Bayesian propotional hazards models. However, the advantages of using the proportional hazards framework, due to the estimation of hazard rates as well as it's application in the competing risk framework are still unassailable. The hazard rate estimation under the censored quantile regression framework is an area that is still under development and the computational aspects are yet to be incorporated into the mainstream statistical softwares. This study concludes that, with the current literature and computational support, using both model frameworks to ascertain the dynamic effects of different prognostic risk factors for survival in people living with HIV/AIDS and on ART would give the researchers more insights. These insights will then help public health policy makers to draft relevant targeted policies aimed at improving these patients' survival time on treatment.

APA, Harvard, Vancouver, ISO, and other styles

36

Choi, Ickwon. "Computational Modeling for Censored Time to Event Data Using Data Integration in Biomedical Research." Case Western Reserve University School of Graduate Studies / OhioLINK, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=case1307969890.

Full text

APA, Harvard, Vancouver, ISO, and other styles

37

Yu, Jianxiong. "Pavement Service Life Estimation And Condition Prediction." See Full Text at OhioLINK ETD Center (Requires Adobe Acrobat Reader for viewing), 2005. http://www.ohiolink.edu/etd/view.cgi?toledo1132896646.

Full text

Abstract:

Dissertation (Ph.D.)--University of Toledo, 2005.
Typescript. "A dissertation [submitted] as partial fulfillment of the requirements of the Doctor of Philosophy degree in Engineering." Bibliography: leaves 69-74.

APA, Harvard, Vancouver, ISO, and other styles

38

Thapa, Ram. "Modeling Mortality of Loblolly Pine Plantations." Diss., Virginia Tech, 2014. http://hdl.handle.net/10919/46726.

Full text

Abstract:

Accurate prediction of mortality is an important component of forest growth and yield prediction systems, yet mortality remains one of the least understood components of the system. Whole-stand and individual-tree mortality models were developed for loblolly pine plantations throughout its geographic range in the United States. The model for predicting stand mortality were developed using stand characteristics and biophysical variables. The models were constructed using two modeling approaches. In the first approach, mortality functions for directly predicting tree number reduction were developed using algebraic difference equation method. In the second approach, a two-step modeling strategy was used where a model predicting the probability of tree death occurring over a period was developed in the first step and a function that estimates the reduction in tree number was developed in the second step. Individual-tree mortality models were developed using multilevel logistic regression and survival analysis techniques. Multilevel data structure inherent in permanent sample plots data i.e. measurement occasions nested within trees (e.g., repeated measurements) and trees nested within plots, is often ignored in modeling tree mortality in forestry applications. Multilevel mixed-effects logistic regression takes into account the full hierarchical structure of the data. Multilevel mixed-effects models gave better predictions than the fixed effects model; however, the model fits and predictions were further improved by taking into account the full hierarchical structure of the data. Semiparametric proportional hazards regression was also used to develop model for individual-tree mortality. Shared frailty model, mixed model extension of Cox proportional hazards model, was used to account for unobserved heterogeneity not explained by the observed covariates in the Cox model.
Ph. D.

APA, Harvard, Vancouver, ISO, and other styles

39

Lukaševičiūtė, Daiva. "Regresiniai modeliai išgyvenamumo analizėje ir jų taikymas ligonių, sergančių reumatoidiniu artritu, mirtingumo analizei." Master's thesis, Lithuanian Academic Libraries Network (LABT), 2010. http://vddb.laba.lt/obj/LT-eLABa-0001:E.02~2009~D_20101125_190734-77321.

Full text

Abstract:

Darbo metu buvo išnagrinėta įvairių faktorių (kovariančių) įtaka reumatoidiniu artritu sergančio 531 ligonio mirtingumui. Buvo taikomas vienas iš regresinių išgyvenamumo modelių – Cox’o modelis. Iš minėtos 531 ligonio imties mirę buvo 32 ligoniai. Iš pradžių buvo tiriama ligonių imtis laiko nuo ligos pradžios aspektu. Šiuo atveju prognozuojantys veiksniai buvo amžius, kada liga buvo diagnozuota (AMZDGN), lytis (LYTKOD), gydymas Metotreksatu (GYD_MTX) ir gydymas Azatriopinu/Imuranu (AZA_IMUR). Vėliau, tiriant ligonių mirtingumą kaip amžiaus funkciją, nustatyti svarbiausi lemiantys veiksniai buvo šie: ligonių lytis (LYTKOD) ir gydymas Azatriopinu/Imuranu (AZA_IMUR). Gauti rezultatai, t.y. ligonių išgyvenamumą lemiančios kovariantės (veiksniai), beveik visiškai sutampa su gydytojų nurodytais. Tai dar kartą patvirtina matematinių statistinių modelių, šiuo atveju nagrinėjamo Cox‘o modelio, taikymo realiame gyvenime, svarbą. Kitai duomenų imčiai, t.y. vėžiu sergančių ligonių duomenų aibei, buvo taikomas Persikertančių mirimų intensyvumų (SCE) modelis, t.y. tikrinama Cox‘o modelio adekvatumo duomenims hipotezė. Hipotezė buvo atmesta, nes minėtiems duomenims Cox‘o modelis negalioja. Pagrindinis darbo rezultatas yra šis: gautas kriterijus Cox‘o modelio adekvatumui tikrinti, naudojant nupjautus iš kairės ir cenzūruotus iš dešinės duomenis, sudarytos programos kriterijui realizuoti. Reumatoidinio artrito ligonių duomenų aibei, t.y. nupjautiems iš kairės ir cenzūruotiems iš dešinės... [toliau žr. visą tekstą]
In this work the Cox proportional hazards model was applied to investigate the influence of various factors (covariates) to mortality of rheumatoid arthritis patients of Vilnius. In the first case, the sample of 531 patients was analysed. Analysing survival of patients of the sample as function of time from the beginnig of the disease, the prognostic factors were LYTKOD (the sex of patients), AMZDGN (patients‘ age, when the rheumatoid arthritis was diagnosed), GYD_MTX (treatment with metotrexat) and AZA_IMUR (treatment with Azatriopin/Imuran). When survival was analysed as function of age then the prognostic factor were LYTKOD (the sex of patients) and AZA_IMUR (treatment with Azatriopin/Imuran). The results are almost identical to those, which doctors suggested. This fact confirms the importance of using mathematical statistical models to solve the problems of the real life. In this case, the importance of using the Cox model. On the other hand, Simple cross-effects (SCE) model was aplied for the sample of canser patients. In the case of this model the hypothesis of Cox model fiting for canser patients‘ data was rejected. The most important result of this work is that the criterion of Cox model fitting to left truncated and right censored data was constructed. Also a program of SAS for the criterion was created. The the hypothesis of Cox model fiting for the rheumatoid arthritis patients wasn‘t rejected, because Cox model fit for these data.

APA, Harvard, Vancouver, ISO, and other styles

40

Sjöström, Lars. "Differences in age at breeding between two genetically different populations of brown trout (Salmo trutta)." Thesis, Uppsala universitet, Statistiska institutionen, 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-388613.

Full text

Abstract:

Survival analysis is an effective tool for conservation studies, since it measure the risk of an event that is important for the survival of populations and preservation of biodiversity. In this thesis three different models for survival analysis are used to estimate the age at breeding between two genetically different populations of brown trout. These populations are an evolutionary enigma, since they apparently coexist in direct competition with each other, which according to ecological theory should not happen. Thus it is of interest if differences between them can be identified. The data consists of brown trouts and has been collected over 20 years. The models are the Cox Proportional Hazards model, the Complementary Log-Log Link model and the Log Logistic Accelerated Failure-Time model. The Cox model were estimated in three different ways due to the nonproportional hazards in the estimates of time to breeding, which gave different interpretations of the same model. All of the models agree that the population B breed at younger ages than the population A, which suggests that the two populations have different reproductive strategies.

APA, Harvard, Vancouver, ISO, and other styles

41

Sauls, Beverly J. "Relative Survival of Gags Mycteroperca microlepis Released Within a Recreational Hook-and-Line Fishery: Application of the Cox Regression Model to Control for Heterogeneity in a Large-Scale Mark-Recapture Study." Scholar Commons, 2013. http://scholarcommons.usf.edu/etd/4940.

Full text

Abstract:

The objectives of this study were to measure injuries and impairments directly observed from gags Mycteroperca microlepis caught and released within a large-scale recreational fishery, develop methods that may be used to rapidly assess the condition of reef fish discards, and estimate the total portion of discards in the fishery that suffer latent mortality. Fishery observers were placed on for-hire charter and headboat vessels operating in the Gulf of Mexico from June 2009 through December 2012 to directly observe reef fishes as they were caught by recreational anglers fishing with hook-and-line gear. Fish that were not retained by anglers were inspected and marked with conventional tags prior to release. Fish were released in multiple regions over a large geographic area throughout the year and over multiple years. The majority of recaptured fish were reported by recreational and commercial fishers, and fishing effort fluctuated both spatially and temporally over the course of this study in response to changes in recreational harvest restrictions and the Deepwater Horizon oil spill. Therefore, it could not be assumed that encounter probabilities were equal for all individual tagged fish in the population. Fish size and capture depth when fish were initially caught-and-released also varied among individuals in the study and potentially influenced recapture reporting probabilities. The Cox proportional hazards regression model was used to control for potential covariates on both the occurrence and timing of recapture reporting events so that relative survival among fish released in various conditions could be compared. A total of 3,954 gags were observed in this study, and the majority (77.26%) were released in good condition (condition category 1), defined as fish that immediately submerged without assistance from venting and had not suffered internal injuries from embedded hooks or visible damage to the gills. However, compared to gags caught in shallower depths, a greater proportion of gags caught and released from depths deeper than 30 meters were in fair or poor condition. Relative survival was significantly reduced (alpha (underline)<(/underline)0.05) for gags released in fair and poor condition after controlling for variable mark-recapture reporting rates for different sized discards among regions and across months and years when individual fish were initially captured, tagged and released. Gags released within the recreational fishery in fair and poor condition were 66.4% (95% C.I. 46.9 to 94.0%) and 50.6% (26.2 to 97.8%) as likely to be recaptured, respectively, as gags released in good condition. Overall discard mortality was calculated for gags released in all condition categories at ten meter depth intervals. There was a significant linear increase in estimated mortality from less than 15% (range of uncertainty, 0.1-25.2%) in shallow depths up to 30 meters, to 35.6% (5.6-55.7%) at depths greater than 70 meters (p < 0.001, R2 = 0.917). This analysis demonstrated the utility of the proportional hazards regression model for controlling for potential covariates on both the occurrence and timing of recapture events in a large-scale mark-recapture study and for detecting significant differences in the relative survival of fish released in various conditions measured under highly variable conditions within a large-scale fishery.

APA, Harvard, Vancouver, ISO, and other styles

42

Alves, Karina Lumena de Freitas. "Análise de sobrevivência de bancos privados no Brasil." Universidade de São Paulo, 2009. http://www.teses.usp.br/teses/disponiveis/18/18140/tde-28102009-103529/.

Full text

Abstract:

Diante da importância do sistema financeiro para a economia de um país, faz-se necessária sua constante fiscalização. Nesse sentido, a identificação de problemas existentes no cenário bancário apresenta-se fundamental, visto que as crises bancárias ocorridas mundialmente ao longo da história mostraram que a falta de credibilidade bancária e a instabilidade do sistema financeiro geram enormes custos financeiros e sociais. Os modelos de previsão de insolvência bancária são capazes de identificar a condição financeira de um banco devido ao valor correspondente da sua probabilidade de insolvência. Dessa forma, o presente trabalho teve como objetivo identificar os principais indicadores característicos da insolvência de bancos privados no Brasil. Para isso, foi utilizada a técnica de análise de sobrevivência em uma amostra de 70 bancos privados no Brasil, sendo 33 bancos insolventes e 37 bancos solventes. Foi possível identificar os principais indicadores financeiros que apresentaram-se significativos para explicar a insolvência de bancos privados no Brasil e analisar a relação existente entre estes indicador e esta probabilidade. O resultado deste trabalho permitiu a realização de importantes constatações para explicar o fenômeno da insolvência de bancos privados no Brasil, bem como, permitiu constatar alguns aspectos característicos de bancos em momentos anteriores à sua insolvência.
The financial system is very important to the economy of a country, than its supervision is necessary. Accordingly, the identification of problems in the banking scenario is fundamental, since the banking crisis occurring worldwide throughout history have shown that and instability of the financial system generates huge financial and social costs. The banking failure prediction models are able to identify the financial condition of a bank based on the value of its probability of insolvency. Thus, this study aimed to identify the main financial ratios that can explain the insolvency of private banks in Brazil. For this, it was used the survival analysis to analize a sample of 70 private banks in Brazil, with 33 solvent banks and 37 insolvent banks. It was possible to identify the key financial indicators that were significantly to explain the bankruptcy of private banks in Brazil and it was possible to examine the relationship between these financial ratios and the probability of bank failure. The result of this work has enabled the achievement of important findings to explain the phenomenon of the bankruptcy of private banks in Brazil, and has seen some characteristic of banks in times prior to its insolvency.

APA, Harvard, Vancouver, ISO, and other styles

43

Calsavara, Vinícius Fernando. "Estimação de efeitos variantes no tempo em modelos tipo Cox via bases de Fourier e ondaletas Haar." Universidade de São Paulo, 2015. http://www.teses.usp.br/teses/disponiveis/45/45133/tde-26082015-140547/.

Full text

Abstract:

O modelo semiparamétrico de Cox é frequentemente utilizado na modelagem de dados de sobrevivência, pois é um modelo muito flexível e permite avaliar o efeito das covariáveis sobre a taxa de falha. Uma das principais vantagens é a fácil interpretação, de modo que a razão de riscos de dois indivíduos não varia ao longo do tempo. No entanto, em algumas situações a proporcionalidade dos riscos para uma dada covariável pode não ser válida e, este caso, uma abordagem que não dependa de tal suposição é necessária. Nesta tese, propomos um modelo tipo Cox em que o efeito da covariável e a função de risco basal são representadas via bases de Fourier e ondaletas de Haar clássicas e deformadas. Propomos também um procedimento de predição da função de sobrevivência para um paciente específico. Estudos de simulações e aplicações a dados reais sugerem que nosso método pode ser uma ferramenta valiosa em situações práticas em que o efeito da covariável é dependente do tempo. Por meio destes estudos, fazemos comparações entre as duas abordagens propostas, e comparações com outra já conhecida na literatura, onde verificamos resultados satisfatórios.
The semiparametric Cox model is often considered when modeling survival data. It is very flexible, allowing for the evaluation of covariates effects. One of its main advantages is the easy of interpretation, as long as the rate of the hazards for two individuals does not vary over time. However, this proportionality of the hazards may not be true in some practical situations and, in this case, an approach not relying on such assumption is needed. In this thesis we propose a Cox-type model that allows for time-varying covariate effects, for which the baseline hazard is based on Fourier series and wavelets on a time-frequency representation. We derive a prediction method for the survival of future patients with any specific set of covariates. Simulations and an application to a real data set suggest that our method may be a valuable tool to model data in practical situations where covariate effects vary over time. Through these studies, we make comparisons between the two approaches proposed here and comparisons with other already known in the literature, where we verify satisfactory results.

APA, Harvard, Vancouver, ISO, and other styles

44

Guedes, Deborah Galv?o Peixoto. "Avalia??o gen?tica da efici?ncia reprodutiva em vacas pardo-su??as por meio da an?lise de sobreviv?ncia." Universidade Federal do Rio Grande do Norte, 2014. http://repositorio.ufrn.br:8080/jspui/handle/123456789/17188.

Full text

Abstract:

Made available in DSpace on 2014-12-17T15:34:49Z (GMT). No. of bitstreams: 1 DeborahGPG_DISSERT.pdf: 1421770 bytes, checksum: da1c0ade32fb8610306243b0cf52f13d (MD5) Previous issue date: 2014-02-20
Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior
Among the traits of economic importance to dairy cattle livestock those related to sexual precocity and longevity of the herd are essential to the success of the activity, because the stayability time of a cow in a herd is determined by their productive and reproductive lives. In Brazil, there are few studies about the reproductive efficiency of Swiss-Brown cows and no study was found using the methodology of survival analysis applied to this breed. Thus, in the first chapter of this study, the age at first calving from Swiss-Brown heifers was analyzed as the time until the event by the nonparametric method of Kaplan-Meier and the gamma shared frailty model, under the survival analysis methodology. Survival and hazard rate curves associated with this event were estimated and identified the influence of covariates on such time. The mean and median times at the first calving were 987.77 and 1,003 days, respectively, and significant covariates by the Log-Rank test, through Kaplan-Meier analysis, were birth season, calving year, sire (cow s father) and calving season. In the analysis by frailty model, the breeding values and the frailties of the sires (fathers) for the calving were predicted modeling the risk function of each cow as a function of the birth season as fixed covariate and sire as random covariate. The frailty followed the gamma distribution. Sires with high and positive breeding values possess high frailties, what means shorter survival time of their daughters to the event, i.e., reduction in the age at first calving of them. The second chapter aimed to evaluate the longevity of dairy cows using the nonparametric Kaplan-Meier and the Cox and Weibull proportional hazards models. It were simulated 10,000 records of the longevity trait from Brown-Swiss cows involving their respective times until the occurrence of five consecutive calvings (event), considered here as typical of a long-lived cow. The covariates considered in the database were age at first calving, herd and sire (cow s father). All covariates had influence on the longevity of cows by Log-Rank and Wilcoxon tests. The mean and median times to the occurrence of the event were 2,436.285 and 2,437 days, respectively. Sires that have higher breeding values also have a greater risk of that their daughters reach the five consecutive calvings until 84 months
Dentre as caracter?sticas de import?ncia econ?mica para a pecu?ria leiteira aquelas relacionadas com a precocidade sexual e a longevidade do rebanho s?o determinantes para o sucesso da atividade, uma vez que o tempo de perman?ncia de uma vaca num rebanho ? determinado por suas vidas produtiva e reprodutiva. No Brasil, existem poucos estudos sobre a efici?ncia reprodutiva de vacas Pardo-Su??as e n?o foi encontrado nenhum estudo abordando o uso da metodologia de an?lise de sobreviv?ncia aplicada a esta ra?a. Assim, no primeiro cap?tulo deste trabalho, a idade ao primeiro parto de novilhas Pardo-Su??as foi analisada como o tempo at? o evento por meio do m?todo n?o-param?trico de Kaplan-Meier e do modelo de fragilidade compartilhado gama, sob a metodologia de an?lise de sobreviv?ncia. Foram estimadas curvas de sobreviv?ncia e de taxa de risco associadas com este evento e verificada a influ?ncia das covari?veis consideradas sobre o tempo. Os tempos m?dio e mediano ao primeiro parto foram 987,77 e 1.003 dias, respectivamente; e as covari?veis significativas pelo teste de Log-Rank, na an?lise por Kaplan-Meier, foram esta??o de nascimento, ano de parto, touro (pai da vaca) e esta??o de parto. Na an?lise pelo modelo de fragilidade, foram preditos os valores gen?ticos e as fragilidades dos touros (pais) para o parto, modelando-se a fun??o de risco de cada vaca em fun??o da covari?vel fixa esta??o de nascimento e da covari?vel aleat?ria touro. A fragilidade seguiu a distribui??o gama. Touros com valores gen?ticos positivos e altos possuem fragilidades altas, o que significa menor tempo de sobreviv?ncia de suas filhas ao evento, ou seja, redu??o na idade ao primeiro parto delas. O segundo cap?tulo teve como objetivo avaliar a longevidade de vacas leiteiras utilizando o estimador n?o-param?trico de Kaplan-Meier e os modelos de riscos proporcionais de Weibull e de Cox. Foram simulados 10.000 registros da caracter?stica longevidade de f?meas Pardo-Su??as denotando seus respectivos tempos at? a ocorr?ncia de cinco partos consecutivos (evento), considerados aqui como t?picos de uma vaca longeva. As covari?veis consideradas no banco de dados foram a idade da vaca ao primeiro parto, o rebanho e o pai da vaca. Todas as covari?veis exerceram influ?ncia sobre a longevidade das vacas de acordo com os testes de Log-Rank e Wilcoxon. Os tempos m?dio e mediano para a ocorr?ncia do evento foram de 2.436,285 e 2.437 dias. Touros com maior valor gen?tico tamb?m possuem um maior risco de que as suas filhas alcancem os cinco partos aos 84 meses

APA, Harvard, Vancouver, ISO, and other styles

45

Heberling, Ulrike, Rainer Koch, Matthias Hübler, Gustavo B. Baretton, Oliver W. Hakenberg, Michael Froehner, and Manfred P. Wirth. "Gender and Mortality after Radical Cystectomy: Competing Risk Analysis." Karger, 2018. https://tud.qucosa.de/id/qucosa%3A70642.

Full text

Abstract:

Background: Data on the impact of gender on mortality after radical cystectomy is conflicting. We investigated a large single center sample with long-term follow-up in order to determine the relationship between gender and outcome. Patients and Methods: A total of 1,184 consecutive patients who underwent radical cystectomy for high risk superficial or muscle-invasive urothelial or undifferentiated bladder cancer between 1993 and 2015 were stratified by gender. Demographic data was compared using Mann-Whitney U test, chi-square test, or Fisher exact test. Cox proportional hazard models were used for the analysis of competing risks and logit models were used for the prediction of the receipt of adjuvant cisplatin-based chemotherapy. Results: Female patients were older, healthier, less frequently current smokers and had more extravesical tumors. In the multivariate analyses, female gender was an independent predictor of (lower) non-bladder cancer (competing) mortality (hazards ratio [HR] 0.68, 95% CI 0.49–0.95, p = 0.0248) but no predictor of bladder cancer-specific mortality (HR in the full model 1.20, 95% CI 0.94–1.54, p = 0.15). Gender was no predictor of the receipt of adjuvant cisplatin-based chemotherapy. Conclusions: Female gender was associated with an increased risk of extravesical disease but was no independent predictor of bladder cancer-specific mortality. Anatomical differences might be a plausible explanation for these observations.

APA, Harvard, Vancouver, ISO, and other styles

46

Bersot, Vitor Fernandes. "Mudança temporal do aleitamento materno exclusivo na América Latina e Caribe: atualização de seus determinantes e da tendência secular." Universidade de São Paulo, 2011. http://www.teses.usp.br/teses/disponiveis/6/6138/tde-28092011-153100/.

Full text

Abstract:

Introdução: Os múltiplos e interativos efeitos protetores do aleitamento materno exclusivo (AME) na saúde e sobrevivência infantil justificam as recomendações universais para promover sua prática. Poucos são os estudos que avaliam a tendência do padrão do AME entre países. Objetivo: Analisar a mudança temporal do AME em cinco países da América Latina e Caribe (ALC) comparando dados das décadas de 1990 e 2000. Métodos: A dissertação é composta por um manuscrito, que avaliou dados de crianças de 0 a 6 meses incluídas nas amostras das pesquisas Demographic Health Survey conduzidas em Brasil, Colômbia, Haiti, Peru e República Dominicana. Foram estimadas as prevalências do AME e suas taxas anuais de variação ponderada, segundo país e ano de inquérito. A duração do AME foi estimada usando a análise de sobrevida de Kaplan-Meier, considerando a idade atual da criança como o tempo de sobrevida e o AME como variável binária, referente à situação da prática no momento da entrevista. As curvas de sobrevivência foram construídas por país, em cada década, e a comparação entre elas usou o teste log-rank. A mediana do tempo de amamentação foi calculada para cada variável independente e a relação entre essas variáveis e o desmame até os seis meses foi analisada pela técnica de regressão de Cox com modelo múltiplo. Resultados: A prevalência de AME aumentou em quatro dos cinco países estudados, com incremento ao ano mais marcante na Colômbia (11 por cento ) e no Haiti (17 por cento ). A duração mediana apresentou duas tendências de evolução: aumento com equidade na Colômbia e no Haiti, e estagnação com distribuição desigual entre os subgrupos populacionais da última década no Brasil, Peru e República Dominicana. No modelo múltiplo de regressão, variáveis de demografia e do perfil de uso dos serviços de saúde associaram-se à duração do AME. A residência em área rural foi a variável reiteradamente associada, de forma negativa no Brasil (HR=1,68; IC 95 por cento :1,06-2,67) e na Colômbia (HR=1,39; IC 95 por cento :1,03-1,87), enquanto que positivamente no Peru (HR=0,40; IC 95 por cento :0,19-0,83). Conclusão: O balanço da tendência do AME na ALC é positivo, embora não uniforme ao longo das duas décadas analisadas. Os achados sinalizam a necessidade de intervenções para a promoção do AME que levem em consideração a localização geográfica das famílias e a qualidade prestada nos serviços de saúde
Introduction: Multiple and interactive protective effects of exclusive breastfeeding (EBF) in health and child survival justify recommendations for promoting universal practice. There are few studies that assess the tendency of the pattern of EBF between countries. Objective: To analyze the temporal change of the AME in five countries in Latin America and Caribbean (LAC) comparing data from 1990 and 2000 decades. Methods: The dissertation consists of a manuscript, which evaluated data from children aged 0 to 6 months in the samples of the Demographic Health Survey conducted research in Brazil, Colombia, Haiti, Peru and the Dominican Republic. Were estimated the prevalence of exclusive breastfeeding and its weighted annual rates of change, according to country and survey year. The duration of EBF was estimated using survival analysis Kaplan-Meier method, considering the current age of the child as the survival time and EBF as binary variable, concerning the state of practice at the time of the interview. The survival curves were constructed for each country, in every decade, and the comparison between them used the log-rank test. The median duration of breastfeeding was calculated for each independent variable and the relationship between these variables and weaning at six months was analyzed using Cox regression model. Results: The prevalence of EBF increased in four of the five countries studied, increasing the most remarkable years in Colombia (II per cent ) and Haiti (17 per cent ). The median duration of evolution showed two trends: growth with equity in Colombia and Haiti, and stagnation with unequal distribution among the population subgroups of the last decade in Brazil, Peru and the Dominican Republic. In the multiple model of regression variables and the demographic profile of use of health services were associated with duration of EBF. The residence in a rural area was the variable consistently associated negatively in Brazil (HR = 1.68, CI 95 per cent : 1,06-2,67) and Colombia (HR = 1.39, CI 95 per cent : 1,03-1,87), while positively in Peru (HR = 0.40, CI 95 per cent : 0,19-0,83). Conclusion: The balance of the trend of EBF in LAC is positive, though not uniform throughout the two decades analyzed. The findings suggest the need for interventions for the promotion of exclusive breastfeeding taking into account the geographical location of families and provided quality health services

APA, Harvard, Vancouver, ISO, and other styles

47

Irobi, Edward Okezie. "Time to Diagnosis of Second Primary Cancers among Patients with Breast Cancer." ScholarWorks, 2016. https://scholarworks.waldenu.edu/dissertations/2661.

Full text

Abstract:

Many breast cancer diagnoses and second cancers are associated with BRCA gene mutations. Early detection of cancer is necessary to improve health outcomes, particularly with second cancers. Little is known about the influence of risk factors on time to diagnosis of second primary cancers after diagnosis with BRCA-related breast cancer. The purpose of this cohort study was to examine the risk of diagnosis of second primary cancers among women diagnosed with breast cancer after adjusting for BRCA status, age, and ethnicity. The study was guided by the empirical evidence supporting the mechanism of action in the mutation of BRCA leading to the development of cancer. Composite endpoint was used to define second primary cancer occurrences, and Kaplan-Meier survival curves were used to compare the median time-to-event among comparison groups and BRCA gene mutation status. Cox proportional hazards was used to examine the relationships between age at diagnosis, ethnicity, BRCA gene mutation status, and diagnosis of a second primary cancer. The overall median time to event for diagnosis of second primary cancers was 14 years. The hazard ratios for BRCA2 = 1.47, 95% CI [1.03 - 2.11], White = 1.511, 95% CI [1.18 - 1.94], and American Indian/Hawaiian = 1.424, 95% CI [1.12 -1.81] showing positive significant associations between BRCA2 mutation status and risk of diagnosis of second primary colorectal, endometrial, cervical, kidney, thyroid, and bladder cancers. Data on risk factors for development of second cancers would allow for identification of appropriate and timely screening procedures, determining the best course of action for prevention and treatment, and improving quality of life among breast cancer survivors.

APA, Harvard, Vancouver, ISO, and other styles

48

Tran, Xuan Quang. "Les modèles de régression dynamique et leurs applications en analyse de survie et fiabilité." Thesis, Bordeaux, 2014. http://www.theses.fr/2014BORD0147/document.

Full text

Abstract:

Cette thèse a été conçu pour explorer les modèles dynamiques de régression, d’évaluer les inférences statistiques pour l’analyse des données de survie et de fiabilité. Ces modèles de régression dynamiques que nous avons considérés, y compris le modèle des hasards proportionnels paramétriques et celui de la vie accélérée avec les variables qui peut-être dépendent du temps. Nous avons discuté des problèmes suivants dans cette thèse.Nous avons présenté tout d’abord une statistique de test du chi-deux généraliséeY2nquiest adaptative pour les données de survie et fiabilité en présence de trois cas, complètes,censurées à droite et censurées à droite avec les covariables. Nous avons présenté en détailla forme pratique deY2nstatistique en analyse des données de survie. Ensuite, nous avons considéré deux modèles paramétriques très flexibles, d’évaluer les significations statistiques pour ces modèles proposées en utilisantY2nstatistique. Ces modèles incluent du modèle de vie accélérés (AFT) et celui de hasards proportionnels (PH) basés sur la distribution de Hypertabastic. Ces deux modèles sont proposés pour étudier la distribution de l’analyse de la duré de survie en comparaison avec d’autre modèles paramétriques. Nous avons validé ces modèles paramétriques en utilisantY2n. Les études de simulation ont été conçus.Dans le dernier chapitre, nous avons proposé les applications de ces modèles paramétriques à trois données de bio-médicale. Le premier a été fait les données étendues des temps de rémission des patients de leucémie aiguë qui ont été proposées par Freireich et al. sur la comparaison de deux groupes de traitement avec des informations supplémentaires sur les log du blanc du nombre de globules. Elle a montré que le modèle Hypertabastic AFT est un modèle précis pour ces données. Le second a été fait sur l’étude de tumeur cérébrale avec les patients de gliome malin, ont été proposées par Sauerbrei & Schumacher. Elle a montré que le meilleur modèle est Hypertabastic PH à l’ajout de cinq variables de signification. La troisième demande a été faite sur les données de Semenova & Bitukov, à concernant les patients de myélome multiple. Nous n’avons pas proposé un modèle exactement pour ces données. En raison de cela était les intersections de temps de survie.Par conséquent, nous vous conseillons d’utiliser un autre modèle dynamique que le modèle de la Simple Cross-Effect à installer ces données
This thesis was designed to explore the dynamic regression models, assessing the sta-tistical inference for the survival and reliability data analysis. These dynamic regressionmodels that we have been considered including the parametric proportional hazards andaccelerated failure time models contain the possibly time-dependent covariates. We dis-cussed the following problems in this thesis.At first, we presented a generalized chi-squared test statisticsY2nthat is a convenient tofit the survival and reliability data analysis in presence of three cases: complete, censoredand censored with covariates. We described in detail the theory and the mechanism to usedofY2ntest statistic in the survival and reliability data analysis. Next, we considered theflexible parametric models, evaluating the statistical significance of them by usingY2nandlog-likelihood test statistics. These parametric models include the accelerated failure time(AFT) and a proportional hazards (PH) models based on the Hypertabastic distribution.These two models are proposed to investigate the distribution of the survival and reliabilitydata in comparison with some other parametric models. The simulation studies were de-signed, to demonstrate the asymptotically normally distributed of the maximum likelihood estimators of Hypertabastic’s parameter, to validate of the asymptotically property of Y2n test statistic for Hypertabastic distribution when the right censoring probability equal 0% and 20%.n the last chapter, we applied those two parametric models above to three scenes ofthe real-life data. The first one was done the data set given by Freireich et al. on thecomparison of two treatment groups with additional information about log white blood cellcount, to test the ability of a therapy to prolong the remission times of the acute leukemiapatients. It showed that Hypertabastic AFT model is an accurate model for this dataset.The second one was done on the brain tumour study with malignant glioma patients, givenby Sauerbrei & Schumacher. It showed that the best model is Hypertabastic PH onadding five significance covariates. The third application was done on the data set given by Semenova & Bitukov on the survival times of the multiple myeloma patients. We did not propose an exactly model for this dataset. Because of that was an existing oneintersection of survival times. We, therefore, suggest fitting other dynamic model as SimpleCross-Effect model for this dataset

APA, Harvard, Vancouver, ISO, and other styles

49

Wu, Bai Rong. "Condition based maintenance using proportional hazards model." Thesis, 2009. http://spectrum.library.concordia.ca/976442/1/MR63089.pdf.

Full text

Abstract:

Condition-based maintenance (CBM) is an advanced maintenance strategy in which maintenance actions are scheduled based on both the age data and condition monitoring information. Proportional Hazards Model (PHM) is a powerful statistical tool for estimating the equipment failure rate under condition monitoring. Effective CBM using PHM can decrease the overall maintenance costs by reducing unnecessary scheduled preventive maintenance actions. In CBM using PHM, main optimization objectives including minimizing maintenance costs and maximizing equipment reliability typically conflict to each other. But the reported research only focuses on single-objective. In this thesis, we propose a multiple-objective CBM optimization approach based on physical programming, which can systematically balance the tradeoff between the optimization objectives and find the optimal solution that best represents the decision maker's preference on the objectives. In CBM using PHM, the accuracy of parameter estimation greatly affects the accuracy of the model in representing and predicting the equipment health condition. Traditional optimization methods such as Newton's methods are inaccurate because they can only find local optimal value in parameter estimation. In this thesis, we develop an approach based on Genetic Algorithms (GA) for PHM parameter estimation and this approach can improve the accuracy of parameter estimation significantly. To illustrate the proposed approaches, we conduct two case studies using real-world vibration monitoring data, shearing pump bearings in a food processing plant and Gould pump bearings at Canadian Kraft Mill. The proposed approaches contribute to the general knowledge of condition based maintenance, and have the potential to greatly benefit various industries.

APA, Harvard, Vancouver, ISO, and other styles

50

Sun, Kang Hua, and 孫康華. "Poor Family in Chia-Yi : Proportional Hazards Model." Thesis, 1998. http://ndltd.ncl.edu.tw/handle/09935935360322373562.

Full text

APA, Harvard, Vancouver, ISO, and other styles

Dissertations / Theses on the topic 'Proportional hazards(PH) model'

Create a spot-on reference in APA, MLA, Chicago, Harvard, and other styles