On 06.03.2010 15:58, Jordi Moya wrote:
Dear friends,
If I use:
model<-lm(y ~ x)
and then extractAIC(model), the value that I obtain does not match (not even
close):
AIC=2*k-2*logLik(model)
However, using AIC from the AICcmodavg(), the value matches exactly the
above value.
I read the help of extractAIC() and could not figure out what was wrong,
other than what I call k (number of parameters) is edf in the help and k
they use for the value 2 that multiplies my k, their edf, and which is
implemeted as the default, thus not possibly being the cause of the
missmatch.
Does anybody know whether am I missing something or whether there may be a
bug in extractAIC()?
?extractAIC tells you
For linear models with unknown scale (i.e., for lm and aov), -2\log L is
computed from the deviance and uses a different additive constant to
logLik and hence AIC.
Hence you want to compare your formula above with
AIC(model)
To compare the code type
stats:::extractAIC.lm
and compare the logLik related devianbce part of it with the
stats:::logLik.lm
as used in AIC().
Best,
Uwe Ligges
Best wishes,
Jordi Moya-Laraño
Cantabrian Institute of Biodiversity (ICAB)
Dpto. BiologÃa de Organismos y Sistemas
Universidad de Oviedo
Catedrático Rodrigo UrÃa s/n
33006-Oviedo
Asturias
Spain
[1]http://www.uniovi.es/icab/jordi.html
References
1. http://www.uniovi.es/icab/jordi.html
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