Two-Part Models for Zero-Modified Count and Semicontinuous Data

  • Brian NeelonEmail author
  • Alistair James O’Malley
Living reference work entry
Part of the Health Services Research book series (HEALTHSR)


Health services data often contain a high proportion of zeros. In studies examining patient hospitalization rates, for instance, many patients will have no hospitalizations, resulting in a count of zero. When the number of zeros is greater or less than expected under a standard count model, the data are said to be zero modified relative to the standard model. More precisely, the data are zero inflated if there is an overabundance of zeros, and zero deflated if there are fewer zeros than expected. A similar phenomenon arises with semicontinuous data, which are characterized by a spike at zero followed by a right-skewed continuous distribution of positive values. When dealing with zero-modified count and semicontinuous data, flexible two-part mixture distributions are often needed to accommodate both the excess zeros and the skewed distribution of nonzero values. A broad array of two-part models has been introduced over the past three decades to accommodate such data. These include hurdle models, zero-inflated models, and two-part semicontinuous models. While these models differ in their distributional assumptions, they each incorporate a two-part structure in which the zero and nonzero observations are modeled in distinct but related ways. This chapter describes recent developments in two-part modeling of zero-modified count and semicontinuous data and highlights their application in health services research.


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Authors and Affiliations

  1. 1.Department of Biostatistics and BioinformaticsDuke University School of MedicineDurhamUSA
  2. 2.Department of Health Care PolicyHarvard Medical SchoolBostonUSA

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