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st: why does -xtnbreg, fe- drop all-zero observations but -xtnbreg, re- does not?


From   "Jason Yackee" <jyackee@law.usc.edu>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: why does -xtnbreg, fe- drop all-zero observations but -xtnbreg, re- does not?
Date   Mon, 5 Feb 2007 17:57:22 -0800

Dear Statalisters,

Would someone be able to explain in plain language why -xtnbreg- with
the "conditional fixed effects" option (-xtnbreg, fe-) drops from the
estimation those i (groups) for whom the dependent (count) variable is
always zero, while -xtnbreg- with a random-effects estimator does not
drop any of those same i (groups)?

(I am aware that for -xtnbreg- "fixed effects" and "random effects" mean
something rather different from the OLS context -- here they refer to
how values for alpha, the over-dispersion parameter, are allocated.  For
both the RE and conditional FE estimators, alpha is constant within
groups).

Much thanks for any explanations... 

 

Jason Webb Yackee, PhD Candidate; J.D.
Fellow, Gould School of Law
University of Southern California
jyackee@law.usc.edu
Cell: 919-358-3040


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