Biomedical subjects

Youyi Shu

Publications and source records attributed to Youyi Shu.

3 recordsLinked to original sources

PubMed2007

Asymptotic theory for the Cox semi-Markov illness-death model.

Irreversible illness-death models are used to model disease processes and in cancer studies to model disease recovery. In most applications, a Markov model is assumed for the multistate model. When there are covariates, a Cox (1972, J Roy Stat Soc Ser B 34:187-220) model is used to model the effect of covariates on each transition intensity. Andersen et al. (2000, Stat Med 19:587-599) proposed a Cox semi-Markov model for this problem. In this paper, we study the large sample theory for that model and provide the asymptotic variances of various probabilities of interest. A Monte Carlo study is conducted to investigate the robustness and efficiency of Markov/Semi-Markov estimators. A real data example from the PROVA (1991, Hepatology 14:1016-1024) trial is used to illustrate the theory.

Biometry↗

PubMed2005

A class of goodness of fit tests for a copula based on bivariate right-censored data.

The copula of a bivariate distribution, constructed by making marginal transformations of each component, captures all the information in the bivariate distribution about the dependence between two variables. For frailty models for bivariate data the choice of a family of distributions for the random frailty corresponds to the choice of a parametric family for the copula. A class of tests of the hypothesis that the copula is in a given parametric family, with unspecified association parameter, based on bivariate right censored data is proposed. These tests are based on first making marginal Kaplan-Meier transformations of the data and then comparing a non-parametric estimate of the copula to an estimate based on the assumed family of models. A number of options are available for choosing the scale and the distance measure for this comparison. Significance levels of the test are found by a modified bootstrap procedure. The procedure is used to check the appropriateness of a gamma or a positive stable frailty model in a set of survival data on Danish twins.

Algorithms↗

PubMed2002

Multi-state models for bone marrow transplantation studies.

High-dose chemotherapy followed by stem cell recovery, more commonly called a bone marrow transplant, is a common treatment for a number of diseases. This article examines four problems commonly encountered when dealing with bone marrow transplant studies. First, we look at the problem of competing causes of failure and at methods based on a multi-state model to estimate meaningful probabilities for these risks. Second, we examine methods for estimating the prevalence of an intermediate condition, here the prevalence of chronic GVHD. Third, we look at the problem of modeling the post transplant recovery process and we provide two examples of how these estimates can be used to assess dynamically a patient's prognosis or how these probabilities can be used to design trials of new therapy. Finally, we present an estimate of a new measure of treatment efficiency, the current leukemia free survival function, which is derived from a multi-state model approach.

Biometry↗