[R-sig-ME] [Lme4-authors] Underdispersion in GLMM

lme4 maintainer bbolker at gmail.com
Mon Mar 31 23:42:23 CEST 2014

  [I'm taking the liberty of forwarding this to r-sig-mixed-models,
where I think it is more appropriate.]

  A few thoughts about underdispersion:

 * underdispersed _count_ data (e.g. litter sizes for large mammals) are
often better captured by an ordinal model (e.g. ordinal::clmm)
 * you _might_ be able to fit underdispersed binomial data by allowing
negative correlation structure within groups: this could be tricky,
though, depending on the size and complexity of your data set.  (The
standard approach of nested random effects is essentially a compound
symmetry structure, but only allows positive/non-negative correlations.
 The nlme package enabled fitting models with a more general CS
structure, but that's not implemented in lme4, and slightly harder to do
in GLMMs in general).
 * the standard 'quasi-likelihood' approach, i.e. taking the estimated
level of underdispersion and shrinking all the confidence intervals
accordingly, might be a reasonable first/hackish approach.  The thing to
be careful about there is that I have yet to read any good treatment (or
figure out for myself) how quasi-likelihood estimates of 'residual'
variance interact with the estimates of the random effects variances ...

  good luck,
  Ben Bolker

On 14-03-31 03:51 PM, Eva Reindl wrote:
> Dear Mr Bates, Mr Maechler, Mr Bolker, Mr Walker, Mr Christensen, and
> Mr Singmann,
> My name is Eva Reindl and I am a Phd student at the University of
> Birmingham working with your lme4 package to analyse my data. I am
> running GLMMs with a binomial error structure and found that my full
> model is heavily underdispersed. I am writing you because I would
> like to ask whether your package offers any strategy to deal with
> underdispersion. Any help or reference to resources would be greatly
> appreciated.
> I would like to thank you very much in advance for your efforts.
> Yours sincerely,
> Eva Reindl
> Eva Reindl MSc Doctoral Researcher at School of Psychology University
> of Birmingham Edgbaston Birmingham B15 2TT United Kingdom
> phone: 0121 414 7209 E-mail: EMR328 at bham.ac.uk 
> _______________________________________________ Lme4-authors mailing
> list Lme4-authors at lists.r-forge.r-project.org 
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