[R-sig-ME] confusion about lmer output

Sacha Viquerat tweedie-d at web.de
Tue Mar 8 22:06:46 CET 2011

hello dear mixed effects modeller! i'm a little confused about the 
output of lmer in R. in michael j. crawleys excellent book on R and in 
various examples in the internet, lmer outputs of poisson family models 
include a dispersion factor (as I gather, the scale of the model). well, 
mine does not show this factor:

Generalized linear mixed model fit by the Laplace approximation
Formula: N_male ~ no3 + (1 | site/sample)
    AIC   BIC logLik deviance
  973.8 987.7 -482.9    965.8
Random effects:
  Groups      Name        Variance Std.Dev.
  sample:site (Intercept) 0.12705  0.35644
  site        (Intercept) 0.43142  0.65683
Number of obs: 243, groups: sample:site, 81; site, 27

Fixed effects:
             Estimate Std. Error z value Pr(>|z|)
(Intercept)   2.2533     0.1434  15.715  < 2e-16 ***
no3          -0.5861     0.1486  -3.944 8.02e-05 ***
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

Correlation of Fixed Effects:
no3 -0.339

in all examples i have seen so far, there's usually a useful bit about 
the dispersion of the model like this:

Estimated scale (compare to 1) 0.9745646

i strongly suspect this to be due to my version of lmer, right? as a 
matter of fact, my only question would be:

how can I tell if my lmer model is overdispersed???


R version 2.11.0 (2010-04-22)

lme4 version 0.999375-38

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