Check this option to include a distribution for either Interventions or Disposition data.Negative binomial fits an extra parameter k that can account for over-dispersion.Note : This option is available only when the Response Type is an incidence density.
Make sure that either Incidence density based on first event only or Incidence density based on all events is specified as the Response Type .Note : If s you see errors regarding NEWRAP convergence in the SAS log after you run an ANOVA report with Negative Binomial selected, you should consider either switching to a Poisson distribution or increasing the value specified in the Gradient Convergence Criterion . This will effectively reduce the restrictive criterion and help you avoid this convergence error.