Working Papers in Economics and Statistics No. 2014-20
We show how the rootogram - a graphical tool associated with the work of J. W. Tukey and originally used for assessing goodness of fit of univariate distributions - can help to diagnose and treat issues such as overdispersion and/or excess zeros in regression models for count data. Two empirical illustrations, from ethology and from public health, are included. The former employs a negative binomial hurdle regression, the latter a two-component finite mixture of negative binomial models for which weighted versions of rootograms are utilized.
rootogram visualization goodness of fit count data Poisson regression negative binomial regression hurdle model finite mixture