Dear list, 

I am needing to extract the estimate of overdispersion (deviance / residual degrees of freedom or c-hat) from multiple model objects - so they can then be used to compare the extent of overdispersion among alternative models as well as calculate qausi-AIC values.  I have been unable to do this, despite consulting a number of manuals and searching the R-help.  I am imaging that in theory it should be possible with some call to attr(), but i have so far had no success.  

An example model output would be: 

> modeltest<-lmer(Coleodactylus_amazonicus_N~USD + (1|site),data=SFArray,family=poisson,method="Laplace",control=list(usePQL=FALSE, msVerbose=TRUE))
> summary(modeltest)

------------

Generalized linear mixed model fit using Laplace 
Formula: Coleodactylus_amazonicus_N ~ USD + (1 | site) 
   Data: SFArray 
 Family: poisson(log link)
      AIC      BIC    logLik deviance
 75.94996 81.68603 -34.97498 69.94996
Random effects:
 Groups Name        Variance Std.Dev.
 site   (Intercept) 2.6076   1.6148  
number of obs: 50, groups: site, 5

Estimated scale (compare to 1)  1.080798 

------------

What I need is to extract this value (1.080798) from multiple lmer objects.  Has anyone any recommendations? I also need to do this for glm objects although I suspect if someone was able to kindly point me in the right direction then the solution is likely to be similar. 

Very many thanks, 

Toby Gardner

> sessionInfo()
Version 2.3.1 (2006-06-01) 
i386-pc-mingw32 

attached base packages:
[1] "datasets"  "graphics"  "grDevices" "methods"   "stats"     "utils"     "base"     

other attached packages:
       JGR     iplots     JavaGD       lme4     Matrix    lattice       MASS      rJava 
   "1.4-7"    "1.0-3"    "0.3-4"  "0.995-2" "0.995-15"   "0.13-8" "7.2-27.1"    "0.4-6" 


School of Environmental Sciences
University of East Anglia
Norwich, NR4 7TJ
United Kingdom
Email: t.gardner@uea.ac.uk
Website: www.uea.ac.uk/~e387495

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