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.
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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.