[R] question about logistic models (AIC)

Ben Bolker bolker at ufl.edu
Fri Aug 3 18:56:53 CEST 2007

Tom Willems <Tom.Willems <at> var.fgov.be> writes:

> I have a question about the Akaike Information Criterion  in the  R 
> output.
> Normally you would want it to be as small as possible, yet when i check up 
> the information in my study books, the AIC is usually displayed as a 
> negative number. In the exercises it is given as " - AIC ".
> R displays it as a positive number, does this mean that a large "AIC" 
> gives a small " - AIC ", so the bigger the better?

  I don't know about your books, but confirm that smaller AIC is better.
AIC is usually positive (likelihood is between 0 and 1,
therefore log-likelihood is negative, therefore -2L + 2k is positive),
although continuous probability densities or neglected normalization
coefficients can lead to negative AICs -- but smaller (more negative,
if AIC<0) is still better.

  Ben Bolker

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