title: Regression analyses of counts and rates: Poisson, overdispersed Poisson, and negative binomial models.
abstract: The regression models appropriate for counted data have seen little use in psychology. This article describes problems that occur when ordinary linear regression is used to analyze count data and presents 3 alternative regression models. The simplest, the Poisson regression model, is likely to be misleading unless restrictive assumptions are met because individual counts are usually more variable ("overdispersed") than is implied by the model. This model can be modified in 2 ways to accomodate this problem. In the overdispersed model, a factor can be estimated that corrects the regression model's inferential statistics. In the second alternative, the negative binomial regression model, a random term reflecting unexplained between-subject differences is included in the regression model. The authors compare the advantages of these approaches.
authors.affiliation: Western Psychiatric Institute and Clinic, University of Pittsburgh School of Medicine, Pennsylvania 15213, USA.
authors.forename: W
authors.initials: W
authors.lastname: Gardner
authors.forename: E P
authors.initials: EP
authors.lastname: Mulvey
authors.forename: E C
authors.initials: EC
authors.lastname: Shaw
mesh_terms: D016010:Binomial Distribution; D003625:Data Collection; D003627:Data Interpretation, Statistical; D006801:Humans; D016014:Linear Models; D016012:Poisson Distribution; D011336:Probability; D012044:Regression Analysis