I'm doing a Zero-Truncated Negative Binomial model in proc fmm. (I'm also considering a Zero-Truncated Poisson although I think we have overdispersion.)
One general question is this. From what I read diving the Pearson Statistic by the degrees of freedom tells you whether there is overdispersion or underdispersion. A ways < 1 implies underdispersion and a ways > 1 implies overdispersion.
Okay, that's one thing. But more specifically, in one model I get the following message in the output:
"Optimization routine cannot improve the function value."
And this same model does not give me results. Any idea what this is? It kinda feels like the "Algorithm did not converge" message I sometimes get in other SAS procs.
Any info is appreciated. Thanks in advance.
