Bias in 2-part mixed models for longitudinal semicontinuous data
Open Access
- 8 January 2009
- journal article
- research article
- Published by Oxford University Press (OUP) in Biostatistics
- Vol. 10 (2), 374-389
- https://doi.org/10.1093/biostatistics/kxn044
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.Keywords
This publication has 22 references indexed in Scilit:
- A Semi‐Parametric Shared Parameter Model to Handle Nonmonotone Nonignorable MissingnessBiometrics, 2009
- A longitudinal study of the effect of disease activity and clinical damage on physical function over the course of psoriatic arthritis: Does the effect change over time?Arthritis & Rheumatism, 2007
- Adolescent Substance Use Outcomes in the Raising Healthy Children Project: A Two-Part Latent Growth Curve Analysis.Journal of Consulting and Clinical Psychology, 2005
- Asymptotic Bias in the Linear Mixed Effects Model Under Non-Ignorable Missing Data MechanismsJournal of the Royal Statistical Society Series B: Statistical Methodology, 2004
- Marginal models for zero inflated clustered dataStatistical Modelling, 2004
- Repeated measures with zerosStatistical Methods in Medical Research, 2002
- A Two-Part Random-Effects Model for Semicontinuous Longitudinal DataJournal of the American Statistical Association, 2001
- Joint modelling of longitudinal measurements and event time dataBiostatistics, 2000
- Comparison of Several Independent Population Means When Their Samples Contain Log‐Normal and Possibly Zero ObservationsBiometrics, 1999
- Estimation and Comparison of Changes in the Presence of Informative Right Censoring by Modeling the Censoring ProcessBiometrics, 1988