[R-sig-ME] Poisson mixed models
Martin Henry H. Stevens
HStevens at muohio.edu
Tue Oct 21 12:19:23 CEST 2008
So you tried a GLMM with quasipoisson and a GLM with Poisson? How
about a GLMM with Poisson? Sounds like you may have a random effect
that is necessary for your hypothesis test, but which does not
explain any variation (but I really have no way of knowing).
On Oct 21, 2008, at 5:33 AM, Renwick, A. R. wrote:
> Dear All
> There has been a lot of talk recently on this forum regarding (over)
> dispersion and quasi models. I am running a GLMM with a poisson
> family for some tick burden data I have and I wanted to check if I
> had overdispersion in my model (and thus a poisson family would be
> inappropriate). The only method I have found to do this is to run
> the model with a quasipoisson family and then ask for the scale
> parameter using:
> However, when I do this my model appears severely UNDER dispersed:
> Without the random effect in the model (i.e a GLM) the scale
> parameter is 1.07 - almost perfect for a poisson family. Is the
> method I am trying not appropriate to determine the dispersion in
> the mixed model? Does anyone know a better method?
> Many thanks,
> The University of Aberdeen is a charity registered in Scotland, No
> R-sig-mixed-models at r-project.org mailing list
Dr. Hank Stevens, Associate Professor
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