[R-sig-ME] Poisson mixed models

Renwick, A. R. a.renwick at abdn.ac.uk
Tue Oct 21 11:33:13 CEST 2008

 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,

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