Journal
STATISTICS IN MEDICINE
Volume 33, Issue 28, Pages 4891-4903Publisher
WILEY
DOI: 10.1002/sim.6263
Keywords
health-care expenditures; log-skew-normal distribution; marginalized models; semicontinuous data; two-part model; weight loss intervention
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Funding
- Diabetes QUERI
- National Center for Health Promotion and Disease Prevention within the Department of VA
- Office of Research and Development, Health Services Research and Development Service, Department of Veterans Affairs
- VA [RCS 10-391, IIR 10-159]
- Daiichi Sankyo
- ResDAC at the University of Minnesota
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In health services research, it is common to encounter semicontinuous data characterized by a point mass at zero followed by a right-skewed continuous distribution with positive support. Examples include health expenditures, in which the zeros represent a subpopulation of patients who do not use health services, while the continuous distribution describes the level of expenditures among health services users. Semicontinuous data are typically analyzed using two-part mixture models that separately model the probability of health services use and the distribution of positive expenditures among users. However, because the second part conditions on a non-zero response, conventional two-part models do not provide a marginal interpretation of covariate effects on the overall population of health service users and non-users, even though this is often of greatest interest to investigators. Here, we propose a marginalized two-part model that yields more interpretable effect estimates in two-part models by parameterizing the model in terms of the marginal mean. This model maintains many of the important features of conventional two-part models, such as capturing zero-inflation and skewness, but allows investigators to examine covariate effects on the overall marginal mean, a target of primary interest in many applications. Using a simulation study, we examine properties of the maximum likelihood estimates from this model. We illustrate the approach by evaluating the effect of a behavioral weight loss intervention on health-care expenditures in the Veterans Affairs health-care system. Copyright (C) 2014 John Wiley & Sons, Ltd.
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