[R-sig-ME] lme4 with Poisson

Douglas Bates bates at stat.wisc.edu
Fri Aug 31 20:03:45 CEST 2012

On Thu, Aug 30, 2012 at 5:25 PM, Lynne Clay <lynne.clay at xtra.co.nz> wrote:
> Dear Prof Bates,
> I'm a doctoral candidate in NZ trying to analyse survey data with random
> effects with my outcome being a count.  I discovered your lme4 package and
> have been using this with success, however, I need to check for
> overdispersion and it is at this point I am having problems.  The formula I
> have used before has been (1/df)*deviance and if I use this my model is
> highly overdispersed.  I read on one of the discussion boards that adding an
> extra random effect (1|id#) addresses the overdispersion problem which I
> have included but overdispersion continues.
> Can overdispersion be calculated in this manner?

I'm sorry but I know nothing about overdispersion.  To me it is
completely artificial because there is no probability distribution on
which to base a statistical model with these properties.

> Do you have any suggestions of how to deal with this?

Sorry but I don't.  I have taken the liberty of sending a copy of this
reply to the R-SIG-Mixed-Models mailing list in the hope that readers
of that list can help you.
> Lynne
> Lynne Clay
> PhD Candidate
> School of Physiotherapy
> University of Otago
> PO Box 56
> Dunedin 9054
> New Zealand

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