[R] AIC, AICc, and K

Spencer Graves spencer.graves at pdf.com
Sun Dec 5 02:09:26 CET 2004

      I don't know the "best" way, but the following looks like it will 

tstDF <- data.frame(x=1:3, y=c(1,1,2))
fit0 <- lm(y~1, tstDF)
fitDF <- lm(y~x, tstDF)
      df      AIC
fitDF  3 5.842516
fit0   2 8.001399

      The function AIC with only 1 argument returns only a single 
number.  However, given nested models, it returns a data.frame with 
colums df and AIC.  At least in this example (and I would think in all 
other contexts as well), "df" is the K you want. 
      hope this helps. 
      Spencer Graves

Benjamin M. Osborne wrote:

>How can I extract K (number of parameters) from an AIC calculation, both to
>report K itself and to calculate AICc?  I'm aware of the conversion from AIC ->
>AICc, where AICc = AIC + 2K(K+1)/(n-K-1), but not sure of how K is calculated
>or how to extract that value from either an AIC or logLik calculation.
>This is probably more of a basic statistics question than an R question, but I
>thank you for your help.
>-Ben Osborne
>Botany Department
>University of Vermont
>109 Carrigan Drive
>Burlington, VT 05405
>benjamin.osborne at uvm.edu
>phone: 802-656-0297
>fax: 802-656-0440
>R-help at stat.math.ethz.ch mailing list
>PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html

Spencer Graves, PhD, Senior Development Engineer
O:  (408)938-4420;  mobile:  (408)655-4567

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