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- Title
Bias in 2-part mixed models for longitudinal semicontinuous data.
- Authors
Li Su; Brian D. M. Tom; Vernon T. Farewell
- Abstract
Semicontinuous data in the form of a mixture of zeros and continuously distributed positive values frequently arise in biomedical research. Two-part mixed models with correlated random effects are an attractive approach to characterize the complex structure of longitudinal semicontinuous data. In practice, however, an independence assumption about random effects in these models may often be made for convenience and computational feasibility. In this article, we show that bias can be induced for regression coefficients when random effects are truly correlated but misspecified as independent in a 2-part mixed model. Paralleling work on bias under nonignorable missingness within a shared parameter model, we derive and investigate the asymptotic bias in selected settings for misspecified 2-part mixed models. The performance of these models in practice is further evaluated using Monte Carlo simulations. Additionally, the potential bias is investigated when artificial zeros, due to left censoring from some detection or measuring limit, are incorporated. To illustrate, we fit different 2-part mixed models to the data from the University of Toronto Psoriatic Arthritis Clinic, the aim being to examine whether there are differential effects of disease activity and damage on physical functioning as measured by the health assessment questionnaire scores over the course of psoriatic arthritis. Some practical issues on variance component estimation revealed through this data analysis are considered.
- Subjects
MONTE Carlo method; ARTHRITIS; FEASIBILITY studies; LONGITUDINAL method
- Publication
Biostatistics, 2009, Vol 10, Issue 2, p374
- ISSN
1465-4644
- Publication type
Article
- DOI
10.1093/biostatistics/kxn044