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Re: st: offset in zip regression model


From   Richard Williams <Richard.A.Williams.5@ND.edu>
To   statalist@hsphsun2.harvard.edu, <statalist@hsphsun2.harvard.edu>
Subject   Re: st: offset in zip regression model
Date   Wed, 30 Jul 2008 09:55:27 -0500

At 08:35 AM 7/30/2008, Jen Rose wrote:
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.
I don't know if it is appropriate or not, but you might be able to use the constraints option. Something like

constraint 1 [count]offset=1
zip numtxdays cond offset, inflate(cond, offset(offset)) vce(cluster rctr) vuong constraint(1)


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Richard Williams, Notre Dame Dept of Sociology
OFFICE: (574)631-6668, (574)631-6463
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EMAIL: Richard.A.Williams.5@ND.Edu
WWW: http://www.nd.edu/~rwilliam

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