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Re: st: Nested zip

From   Richard Williams <>
Subject   Re: st: Nested zip
Date   Tue, 06 Dec 2011 13:56:17 -0500

At 12:13 PM 12/6/2011, Kris Anderson wrote:

I am trying to model a count variable (lifetime use of drugs) nested in schools. Given it's distribution, a zip model would be ideal (lots of zeros). However, it seems Stata will run an xtmepoisson but not zip. Does anyone know why?


In Paul Allison's book "Fixed Effects Regression Models" he says (p. 68) that he does not consider zero-inflated Poisson and negative binomial models because (1) they are more complex (2) there is little software available for panel data (3) the negative binomial model itself often provides a satisfactory fit to data with large numbers of zero counts.

As a sidelight, he also argues (pp. 62-64) that the method used by -xtnbreg- is not a true fixed effects method and gives examples of where he claims xtnbreg gives nonsensical results. He offers some alternative (albeit time-consuming) approaches. I'd be curious to hear if StataCorp agrees or disagrees with Allison's critique. The book is from 2009 but repeats an argument he made in a 2002 article.

Richard Williams, Notre Dame Dept of Sociology
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