Douglas Bates bates at stat.wisc.edu
Tue Mar 8 23:19:25 CET 2011

```On Tue, Mar 8, 2011 at 3:06 PM, Sacha Viquerat <tweedie-d at web.de> wrote:
> 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:
>    (Intr)
> 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:

Right.

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

I'll let others more familiar with overdispersion than am I answer that.

> info:
>
> R version 2.11.0 (2010-04-22)
>
> lme4 version 0.999375-38

Both of those are out-of-date.  If possible i would recommend
upgrading to R-2.12.2 and to lme4_0.999375-39

>
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>

```