# Differences for PROC GENMOD, COUNTREG and FMM for count data model

I want to fit a negative binomial model for my count data. I am wondering what the differences for the computation of these three PROCs are in SAS.

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On the face of it, this question ought to have a routine answer in the SAS manuals and help files. Is there something in those references that you need help with? –  whuber Aug 10 '12 at 15:26
@whuber, I just a little bit confused on how the manual describes this. They seems to perform the same task and I am not sure if the calculation would be different. –  Seen Aug 10 '12 at 18:49
@Seen I think you would get a more helpful answer if you could give more details in your question. There are many possible differences between these procedures, if you explain what is confusing or what specifically you are interested in, we can give a more focused answer. –  Joshua Aug 11 '12 at 2:40

There are many overlapping procedures in SAS. If all you want to do is fit a negative binomial model, any of those should be fine. Differences arise in that each of those procedures has additional abilities. For example, proc genmod has flexible residual correlation structures, proc countreg offers bounds and constraint options, proc fmm fits finite mixture models, which are a very flexible class of models but it has less post estimation capacities built in.
If a procedure does everything you want, there is probably little reason to use a different one. However if it does not or it is difficult, there may be. For example proc genmod has an estimate statement whereas proc countreg does not, although one could still get estimates by output the parameter estimates and computing them manually.