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st: RE: Adding "offset" to ZINB and ZIP models


From   "Cobb, Adam" <adamcobb@umich.edu>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: RE: Adding "offset" to ZINB and ZIP models
Date   Thu, 21 Oct 2010 09:21:38 -0400

Hi all,

I have a general question regarding zero-inflated negative binomial (ZINB) and poisson regressions (ZIP).  Note:  I use Stata 11 on a Windows machine. 

I am attempting to model the number of employees that participate in a health care plan for a thousand or so companies over several years.  I was exploring ZINB models because several firms do not have such a plan (or abandon it during the period of study) so there are a lot of 0 (but not missing) observations.  In talking with some people, a few indicated I should use the "offset" to offset the number of employees in the firm.  The rationale being that the firm can't have more participants in the plan than they have employees.  However, I didn't think that is what the offset was for and a a few other people I spoke with didn't believe that is what it was for either.  The stata documentation states that "offset" constrains the offset variable to have a coefficient of 1, but I'm not 100% sure how this would affect a model.   Basically I'm just looking for information regarding adding an "offset" variable to a model.  If anyone has any insight they can provide on using an "off!
 set" in a ZIP or ZINB model, an example of when an "offset" is necessary, etc ... I'd be most thankful.  

Sincerely,

Adam 
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