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The Stata Journal
Volume 14 Number 2: pp. 280-291



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Regression models for count data based on the negative binomial(p) distribution

James W. Hardin
Institute for Families in Society
Department of Epidemiology and Biostatistics
University of South Carolina
Columbia, SC
[email protected]
Joseph M. Hilbe
School of Social and Family Dynamics
Arizona State University
Tempe, AZ
[email protected]
Abstract.  We present new Stata commands for estimating several regression models suitable for analyzing overdispersed count outcomes. The nbregp command nests the dispersion(constant) and dispersion(mean) versions of Stata’s nbreg command in a model for negative binomial(p) regression. The zignbreg command extends Stata’s gnbreg command for zero inflation, and the zinbregp command fits a negative binomial(p) regression model with zero inflation. The new commands for zero-inflated models allow specification of links within the glm command’s collection for the Bernoulli model of zero inflation. These commands will optionally calculate a Vuong test, which compares the zero-inflated model with the nonzeroinflated model.
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