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st: "Regular" fixed effects and nbreg, in stata8.2?


From   SamL <saml@demog.berkeley.edu>
To   Stata Listserve <statalist@hsphsun2.harvard.edu>
Subject   st: "Regular" fixed effects and nbreg, in stata8.2?
Date   Tue, 7 Jun 2005 07:23:38 -0700 (PDT)

The manual states that for the command -xtnbreg- with the -fe- option, the
term "fixed effects" applies "to the distribution of the dispersion
parameter, and not to the xB term in the model."  I am understanding this
to mean that there is no fixed effect--in the usual sense of a beta
coefficient for the unit (i) to which the observation belongs.  I have two
questions.

1)Is my understanding correct?

2)If my understanding is correct, may one obtain a fixed effects nbreg
model that *does* have a fixed effect, in the usual xB sense, for the unit
to which each observation belongs and, if so, how?  (I don't need to see
the fixed effect--if it drops out of the conditional model, that's fine--I
just need for the usual fixed effect to drop out, not the dispersion
parameter. For my analysis an equal dispersion parameter across units
would be fine.).

There are other complications, but the gist is above.  Those complications
include:

a)needing to use the noconstant option, because I am estimating a very
small set of group-specific means (to be consistent with other analyses of
other outcomes in the same project--this makes it impossible for me to use
-xtpoisson, fe-)

b)wanting to allow the cluster option to reflect clustering on *another*
variable with respect to the standard errors,

and c)ideally, being able to estimate a two-way fixed effect model--but
that may not be possible.

Of these, a is the most important.

Thanks for any assistance you can provide.
Sam
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