[R-sig-ME] Poisson mixed models

Martin Henry H. Stevens HStevens at muohio.edu
Tue Oct 21 12:19:23 CEST 2008

Hi Anna,
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:
> lme4:::sigma(model)
> However, when I do this my model appears severely UNDER dispersed:
>  sigmaML
> 3.779694e-06
> 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,
> Anna
> The University of Aberdeen is a charity registered in Scotland, No  
> SC013683.
> _______________________________________________
> R-sig-mixed-models at r-project.org mailing list
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