[R] AIC in lmer

[Douglas Bates]

The calculation is being done in

                  print(data.frame(AIC = AIC(llik), BIC = BIC(llik),
                               logLik = c(llik),
                               deviance = -2*llik,
                               row.names = ""))

where llik is defined as

              llik <- object at logLik

so the important question is whether the logLik slot has the correct
values for the number of parameters.

By the way, why are you calculating a random effect for year when you
only have two years of data? The estimate of the variance of that
random effect will have almost no precision.

On 10/7/05, Richard Chandler <rchandler at forwild.umass.edu> wrote:
> Hello all,
>
> Is AIC calculated incorrectly in lmer? It appears as though it uses
> AIC = -2*logLik - 2*#parms, instead of -2*LogLik + 2*#parms? Below is
> output from one of many models I have tried:
>
> Generalized linear mixed model fit using PQL
> Formula: cswa ~ pcov.ess1k + (1 | year)
>    Data: ptct50.5
>  Family: poisson(log link)
>       AIC    BIC    logLik deviance
>  224.8466 219.19 -114.4233 228.8466
> Random effects:
>      Groups        Name    Variance    Std.Dev.
>        year (Intercept)   0.0062643    0.079147
> # of obs: 125, groups: year, 2
>
> Estimated scale (compare to 1)  1.277183
>
> Fixed effects:
>               Estimate Std. Error  z value Pr(>|z|)
> (Intercept) -0.1059628  0.1283976 -0.82527   0.4092
> pcov.ess1k   0.0101182  0.0093962  1.07683   0.2816
>
>
> A snip of my data:
>
>       cswa pcov.ess250 year
>   [1,]    4        7.14 2004
>   [2,]    4       19.26 2003
>   [3,]    1        3.66 2004
>
> I'm using R 2.1.1 with Windows XP.
>
> Thanks,
> Richard
>
> --
> Richard Chandler
> Department of Natural Resources Conservation
> UMass Amherst
> (413)545-1237
>
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