[R] Problem comparing Akaike's AIC - nlme package

From: sbegueria <sbegueria_at_eead.csic.es>
Date: Tue, 11 Mar 2008 12:17:52 -0700 (PDT)

Hello,

I am comparing models made with nlme functions and non-nlme functions, based on Akaike's AIC. The AIC values I get for exactly the same model formulation --for example a linear model with no random effects fit with gls and lm, respectively-- do not fit, although the values of the four model parameters are exactly the same. For example:

m1 <- gls(height ~ age, data = Loblolly) m2 <- lm(height ~ age, data = Loblolly)

m1$coefficients
(Intercept) age

  -1.312396 2.590523
m2$coefficients
(Intercept) age

  -1.312396 2.590523

But then:

AIC(m1)
[1] 428.9243
AIC(m2)
[1] 423.9153

I am trying to compare between more complex models, i.e. different ways of incorporating spatial self-correlation, and this issue with the AIC is really making me silly!

Thanks,

    S. Begueria

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