Accounting for Excess Zeros and Sample Selection in Poisson and Negative Binomial Regression Models
William H. Greene
New York University Stern School of Business
NYU Working Paper No. EC-94-10
We present several modifications of the Poisson and negative binomial models for count data to accommodate cases in which the number of zeros in the data exceed what would typically be predicted by either model. The excess zeros can masquerade as overdispersion. We present a new test procedure for distinguishing between zero inflation and overdispersion. We also develop a model for sample selection which is analogous to the Heckman style specification for continuous choice models. An application is presented to a data set on consumer loan behavior in which both of these phenomena are clearly present.
Number of Pages in PDF File: 37
Date posted: November 3, 2008
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