Hi,
I�m wondering if anyone can tell me the correct way to use an offset
variable in a zero inflated poisson regression model. I�ve used the
following command which seems to test the offset variable as a component of
the zero inflation part of the outcome variable, but not the count part of
the outcome:
zip numtxdays cond, inflate(cond, offset(offset)) vce(cluster rctr) vuong
My question is whether or not there�s a way to account for the offset
variable while testing the count part of the model in Stata, and if so, is
it appropriate to do so? The reason I ask is that, if I use SAS PROC NLMIXED
or Mplus, I can include the offset variable in the test of the count part of
the outcome variable, as well. If I do, I get radically different results
than if it is only accounted for in the inflation part of the model. �
Thanks for your help - any advice is greatly appreciated,
Jen Rose
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