title: Mixtures of varying coefficient models for longitudinal data with discrete or continuous nonignorable dropout.
abstract: The analysis of longitudinal repeated measures data is frequently complicated by missing data due to informative dropout. We describe a mixture model for joint distribution for longitudinal repeated measures, where the dropout distribution may be continuous and the dependence between response and dropout is semiparametric. Specifically, we assume that responses follow a varying coefficient random effects model conditional on dropout time, where the regression coefficients depend on dropout time through unspecified nonparametric functions that are estimated using step functions when dropout time is discrete (e.g., for panel data) and using smoothing splines when dropout time is continuous. Inference under the proposed semiparametric model is hence more robust than the parametric conditional linear model. The unconditional distribution of the repeated measures is a mixture over the dropout distribution. We show that estimation in the semiparametric varying coefficient mixture model can proceed by fitting a parametric mixed effects model and can be carried out on standard software platforms such as SAS. The model is used to analyze data from a recent AIDS clinical trial and its performance is evaluated using simulations.
authors.affiliation: Center for Statistical Sciences, Department of Community Health, Brown University, Box G-H, Providence, Rhode Island 02912, USA. jwh@brown.edu
authors.forename: Joseph W
authors.initials: JW
authors.lastname: Hogan
authors.forename: Xihong
authors.initials: X
authors.lastname: Lin
authors.forename: Benjamin
authors.initials: B
authors.lastname: Herman
mesh_terms: D019380:Anti-HIV Agents / Q000008:administration & dosage; D001699:Biometry; D018791:CD4 Lymphocyte Count; D002648:Child; D003627:Data Interpretation, Statistical; D015658:HIV Infections / Q000188:drug therapy / Q000276:immunology; D006801:Humans; D016014:Linear Models; D008137:Longitudinal Studies; D015233:Models, Statistical; D016032:Randomized Controlled Trials as Topic / Q000706:statistics & numerical data; D015215:Zidovudine / Q000008:administration & dosage
chemical_list: D019380:Anti-HIV Agents; D015215:Zidovudine