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st: offset in zip regression model


From   "Jen Rose" <jrose01@wesleyan.edu>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: offset in zip regression model
Date   Wed, 30 Jul 2008 09:35:30 -0400

Hi,

Iím wondering if anyone can tell me the correct way to use an offset
variable in a zero inflated poisson regression model. Iíve 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 thereís 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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