Re: [R] AIC in lmer

From: vito muggeo <vmuggeo_at_dssm.unipa.it>
Date: Sat 08 Oct 2005 - 01:23:00 EST

Hi,
my reply just concerns the usage of AIC in mixed models and not the lmer package.

The "standard" AIC is actually unconditional.

Vaida and Blanchard (2003, Proceeding 19 IWSM,101-105) discuss that a "conditional" version should be more appropriate in a mixed framework.

I don't whether the paper has been pubblished elsewhere.

regards,
vito

Richard Chandler 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
>

-- 
====================================
Vito M.R. Muggeo
Dip.to Sc Statist e Matem `Vianelli'
UniversitÓ di Palermo
viale delle Scienze, edificio 13
90121 Palermo - ITALY
tel: 091 6626240
fax: 091 485726/485612

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Received on Sat Oct 08 01:27:55 2005

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