[R] AIC models are not all fitted to the same number of observation
Patrick Giraudoux
patrick.giraudoux at univ-fcomte.fr
Wed Mar 21 10:56:05 CET 2012
Hi,
Using lme from the package nlme 3.1-103, I meet a strange warning. I am
trying to compare to models with:
library(nlme)
lmez6=lme(lepus~vulpes,random=~1|troncon/an,data=ika_z6_test)
lmez60=lme(lepus~1,random=~1|troncon/an,data=ika_z6_test)
Both have the same number of observations and groups:
lmez6
Linear mixed-effects model fit by REML
Data: ika_z6_test
Log-restricted-likelihood: -2267.756
Fixed: lepus ~ vulpes
(Intercept) vulpes
1.35017117 0.04722338
Random effects:
Formula: ~1 | troncon
(Intercept)
StdDev: 0.8080261
Formula: ~1 | an %in% troncon
(Intercept) Residual
StdDev: 1.086611 0.4440076
Number of Observations: 1350
Number of Groups:
troncon an %in% troncon
169 1350
> lmez60
Linear mixed-effects model fit by REML
Data: ika_z6_test
Log-restricted-likelihood: -2266.869
Fixed: lepus ~ 1
(Intercept)
1.435569
Random effects:
Formula: ~1 | troncon
(Intercept)
StdDev: 0.8139646
Formula: ~1 | an %in% troncon
(Intercept) Residual
StdDev: 1.086843 0.4445815
Number of Observations: 1350
Number of Groups:
troncon an %in% troncon
169 1350
...but when I want to compare their AIC, I get:
AIC(lmez6,lmez60)
df AIC
lmez6 5 4545.511
lmez60 4 4541.737
Warning message:
In AIC.default(lmez6, lmez60) :
models are not all fitted to the same number of observations
Has anybody an explanation about this strange warning ? To what extent
this warning may limit the conclusions that could be drawn from AIC
comparison ?
Thanks in advance,
Patrick
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