I believe Elston et al. 2001
(link<http://abdn.ac.uk/lambin-group/Papers/Paper%2041%202001%20Elston%20Tick%20aggregation%20Parasitology.pdf>)
is usually cited for using observation-level random effects, but is appears
to be treated skeptically by some
(link<https://stat.ethz.ch/pipermail/r-sig-mixed-models/2011q1/015867.html>),
at least for some uses of it.

Would glmmPQL in the MASS package work for your purposes, using a
quasipoisson distribution? It appears to be consistent with Bolker et al.
2009 (link<http://facecouncil.org/puf/wp-content/uploads/2009/10/Bolker-et-al-2009-TREE.pdf>)
recommendations (Fig. 1 for quick summary). Apparently glmmPQL
automatically estimates the overdispersion parameter
(link<http://books.google.com/books?id=vQUNprFZKHsC&pg=PA332&lpg=PA332&dq=glmmpql+overdispersion&source=bl&ots=kayJyV2C_l&sig=d6IrucMKMbKDD7V6I2zb4qFeq9I&hl=en&sa=X&ei=XTaWUobyNa6-sQSJjIDgDQ&ved=0CFYQ6AEwBA#v=onepage&q=glmmpql%20overdispersion&f=false>
).

I'd be curious to hear from others if whether there are inherent flaws in
this approach. I'm not terribly experienced in this area but this was the
solution I came upon when I dealt with this issue myself.

Brad


On Tue, Nov 26, 2013 at 5:00 AM, Enrico Caprio <enrico.caprio@gmail.com>wrote:

> Dear R-ecologists,
> I am running several mixed models (using glmer function of lme4 library) of
> count data (species richness and abundances).
> My data are spiders and ground beetles trapped in pitfalls in different
> locations and habitats and in different periods during a season.
>
> The best approach to analyze them for me was building a mixed model, using
> the pitfall in each location, the locations and the periods as random
> factor.  Habitat type and habitat variables are fixed factors.
>
> This approach deals well with overdispersion in most of the models, but in
> few cases I still have overdispersion. I read in forums and learned at some
> stat courses that a way to deal with overdispersion is using the
> observation level as  an extra random factor and this works fine but I
> would like to read something more about this and need some references.
>
> Do you know a paper or book that can help me with that?
>
> Thanks!
>
> Enrico
>
> --
> Enrico CAPRIO, PhD
> Laboratorio di Ecologia Animale
> Dipartimento di Scienze della Vita e Biologia dei Sistemi
> Via Accademia Albertina, 13
> I - 10123 Torino
> Cell. 3332134420
> http://www.personalweb.unito.it/enrico.caprio/
>
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>
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>



-- 

Bradley Evan Carlson
PhD Candidate
Intercollege Graduate Degree Program in Ecology
The Pennsylvania State University
University Park, PA 16802

Email: carbrae@gmail.com
http://www.personal.psu.edu/tll30/index_files/carlson.htm<http://homes.bio.psu.edu/people/faculty/langkilde/index_files/carlson.htm>
https://sites.google.com/site/bradleyecarlson/home

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