Modeling Semicontinuous Longitudinal Expenditures: A Practical Guide.

OBJECTIVE: To compare different strategies for analyzing longitudinal expenditure data that have a point mass at $0. We provide guidance on parameter interpretation, research questions, and model selection.

DATA SOURCES, STUDY DESIGN, AND DATA COLLECTION: One-part models, uncorrelated two-part models, correlated conditional two-part (CTP) models, and correlated marginalized two-part (MTP) models have been proposed for longitudinal expenditures that often exhibit a large proportion of zeros and a distribution of continuous, highly right-skewed positive values. Guidance on implementing and interpreting each of these model is illustrated with an example of longitudinal (2000-2003) specialty care expenditures of veterans with hypertension, drawn from Veterans Administration data.

PRINCIPAL FINDINGS: The four strategies answer different research questions, are appropriate for different structures of data, and provide different results. If there is a point mass at $0, then the MTP model may be most useful if the primary interest is in mean expenditures of the entire population. A CTP model may be most useful if the primary interest is in the level of expenditures conditional on them being incurred.

CONCLUSIONS: Researchers should consider which modeling strategy for longitudinal expenditure outcomes is both consistent with research aims and appropriate for the data at hand.