Bayesian generalized linear model for over and under dispersed counts

Abstract: Bayesian models that can handle both over and under dispersed counts are rare in the literature, perhaps because full probability distributions for dispersed counts are rather difficult to construct. This note takes a first look at Bayesian Conway-Maxwell-Poisson generalized linear models that can handle both over and under dispersion yet retain the parsimony and interpretability of classical count regression models. The focus is on providing an explicit demonstration of Bayesian regression inferences for dispersed counts via a Metropolis-Hastings algorithm. We illustrate the approach on two data analysis examples and demonstrate some favourable frequentist properties via a simulation study.