[R] dispersion_parameter_GLMM's

[David Barron]

That amount of overdispersion would make the use of a poisson model
very questionable, and will very likely result in estimated standard
errors that are too low, hence the change in statistical significance
when you switch to quasipoisson.

On 06/03/07, Cristina Gomes <gomes at eva.mpg.de> wrote:
> Hi all,
> I was wondering if somebody could give me advice regarding the
> dispersion parameter in GLMM's. I'm a beginner in R and basically in
> GLMM's. I've ran a GLMM with a poisson family and got really nice
> results that conform with theory, as well with results that I've
> obtained previously with other analysis and that others have obtained in
> similar studies. But the dispersion parameter is too large (25 compared
> to 1). So I ran a quasipoisson that allows for overdispersion and all of
> the significant results fell out. My question is, how much does
> overdispersion invalidate ones results when using a poisson family? What
> could the reason be for having non significant results with a
> quasipoisson distribution?
> If anybody could give me some help with this or/and recommend me
> literature that is somewhat comprehensible to lay people (i.e. non
> statisticians) that would be great.
> Thanks in advance,
> Cristina.
>
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-- 
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David Barron
Said Business School
University of Oxford
Park End Street
Oxford OX1 1HP