title: Structural zeroes and zero-inflated models.
abstract: In psychosocial and behavioral studies count outcomes recording the frequencies of the occurrence of some health or behavior outcomes (such as the number of unprotected sexual behaviors during a period of time) often contain a preponderance of zeroes because of the presence of 'structural zeroes' that occur when some subjects are not at risk for the behavior of interest. Unlike random zeroes (responses that can be greater than zero, but are zero due to sampling variability), structural zeroes are usually very different, both statistically and clinically. False interpretations of results and study findings may result if differences in the two types of zeroes are ignored. However, in practice, the status of the structural zeroes is often not observed and this latent nature complicates the data analysis. In this article, we focus on one model, the zero-inflated Poisson (ZIP) regression model that is commonly used to address zero-inflated data. We first give a brief overview of the issues of structural zeroes and the ZIP model. We then given an illustration of ZIP with data from a study on HIV-risk sexual behaviors among adolescent girls. Sample codes in SAS and Stata are also included to help perform and explain ZIP analyses.
authors.affiliation: Department of Biostatistics and Computational Biology, University of Rochester Medical Center, Rochester, NY, USA ; Veterans Integrated Service Network, Center of Excellence for Suicide Prevention, Canandaigua VA Medical Center, Canandaigua, NY, USA ; Department of Psychiatry, University of Rochester Medical Center, Rochester, NY, USA.
authors.forename: Hua
authors.initials: H
authors.lastname: He
authors.affiliation: Department of Biostatistics and Computational Biology, University of Rochester Medical Center, Rochester, NY, USA.
authors.forename: Wan
authors.initials: W
authors.lastname: Tang
authors.affiliation: Department of Biostatistics and Computational Biology, University of Rochester Medical Center, Rochester, NY, USA.
authors.forename: Wenjuan
authors.initials: W
authors.lastname: Wang
authors.affiliation: Department of Psychiatry, University of Pennsylvania, Philadelphia, PA, USA.
authors.forename: Paul
authors.initials: P
authors.lastname: Crits-Christoph
keywords: count response; random zeroes; structural zeroes; zero-inflated models