Testing difference between two Negative Binomial distributions

I’m not sure what you mean by that. Each NB is directly tied to the data (e.g., observed=pre_nzk and observed=prost_nzk) and each NB has its mean and SD parameterized (e.g., pre_mean, pre_std, etc.) and those parameter estimates are then compared in the deterministic diff_of_means, diff_of_stds, and effect_size.