I really appreciate your reply! Yes, within the cluster Center, my subject IDs are unique. So I removed the nested term following your suggestions. But neither negative binomial nor poisson in Genmod converged using your last suggested models. It is interesting that the Glimmix gets converged even with the nested terms (code in my original post). And I've been trying to understand why and if I could trust the converged results. In addition, does that indicate Glimmix is a better approach for my data compared to Genmod? ... View more