We explore the determinants of usage of six different types of health careservices, using the Medical Expenditure Panel Survey data, years 1996-2000.We apply a number of models for univariate count data, including semiparametric, semi-nonparametric and finite mixture models.We find that the complexity of the model that is required to fit the datawell depends upon the way in which the data is pooled across sexes andover time, and upon the characteristics of the usage measure.Pooling across time and sexes is almost always favored, but when more heterogeneous data is pooled it is often the case that a more complex statisticalmodel is required.This research was supported by grants SEC 2003-05112 and SGR 2001-0164