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AW: st: xtnbreg, fe does not converge when xtpoisson does

From   "Klaus Pforr" <>
To   <>
Subject   AW: st: xtnbreg, fe does not converge when xtpoisson does
Date   Wed, 22 Aug 2012 16:01:24 +0200


xtpoisson und xtnbreg look at different fixed-effects. xtpoisson adds a
fixed effect to Xb, i.e. an additional possible correlated
within-group-invariant independent variable. xtnbreg models a fixed
over-dispersion-factor (in contrast to the simple nbreg-model): <<Here
<random effects> and <fixed effects> apply to the distribution of the
dispersion parameter, not to the x  term in the model.>> (Manual entry for
xtnbreg. This means, that your xtpoisson converges because you have enough
variance for the Xb-fixed-effect, but you may have not enough variance for
the fe for the overdispersion-factor.

Klaus Pforr 


Klaus Pforr
GESIS -- Leibniz Institut für Sozialwissenschaft
Postfach 122155
D - 68072 Mannheim
Tel: +49 621 1246 298
Fax: +49 621 1246 100 

-----Ursprüngliche Nachricht-----
[] Im Auftrag von Maarten Buis
Gesendet: Mittwoch, 22. August 2012 11:41
Betreff: Re: st: xtnbreg, fe does not converge when xtpoisson does

On Tue, Aug 21, 2012 at 8:08 PM, Anastasiya Zavyalova wrote:
> I have a dependent count variable. The range is from 0 to 800,000. 
> When I run an xtpoisson with fixed effects it runs well and, in fact, 
> all my coefficients are highly signifiant.
> However, when I run an xtnbreg with a fixed-effects option (turns out 
> the variable is over-dispersed), I receive the following error
> message: "discontinuous region encountered; cannot compute an 
> improvement; r(430);" What could be the reason? Thank you.

A negative binomial model is a much more complicated model than a Poisson,
so the fact that a Poisson converges does not guarantee that a negative
binomial will converge as well.

-- Maarten

Maarten L. Buis
Reichpietschufer 50
10785 Berlin
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