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Re: st: xtnbreg, nbreg, and tests of assumptions

From   Maarten buis <>
Subject   Re: st: xtnbreg, nbreg, and tests of assumptions
Date   Wed, 15 Dec 2010 18:45:08 +0000 (GMT)

--- On Wed, 15/12/10, Dalhia wrote:
> Here are the results for xtnbreg that don't make sense.
> Basically, I have panel data on hospitals (private, public,
> and associates), and looking at the averages of the number
> of training days for each hospital type, I can see that
> private hospitals have lower number of training days
> compared to public hospitals. Associate hospitals fall in
> the mid-range. However, when I run this model using xtnbreg
> (with random effects), I get a funny result. It looks like
> public and associates have lower rate of training days in a
> year compared to private.

That can happen when countries with more public hospitals spent
a lot more on training. The "country-effect" might then be 
enough to induce a spurious positive effect, even if the within
country effect is actually negative. I would try to graph the
data and assign different markers/colors to different countries
to see the within and between country effects. That is usually
a good way to build an intuition of what is going on in your

Hope this helps,

Maarten L. Buis
Institut fuer Soziologie
Universitaet Tuebingen
Wilhelmstrasse 36
72074 Tuebingen


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