[R] Extracting overdispersion estimates from lmer amd glm objects

[Simon Blomberg]

summary(modeltest)@sigma

Toby Gardner wrote:
> 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 at uea.ac.uk
> Website: www.uea.ac.uk/~e387495
>
> 	[[alternative HTML version deleted]]
>
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>
>   


-- 
Simon Blomberg, B.Sc.(Hons.), Ph.D, M.App.Stat. 
Centre for Resource and Environmental Studies
The Australian National University              
Canberra ACT 0200                               
Australia                                       
T: +61 2 6125 7800 email: Simon.Blomberg_at_anu.edu.au
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The combination of some data and an aching desire for 
an answer does not ensure that a reasonable answer 
can be extracted from a given body of data.
- John Tukey.