[R] help about how can R compute AIC?
Bernardo Rangel Tura
tura at centroin.com.br
Wed Oct 15 11:14:12 CEST 2008
Em Ter, 2008-10-14 às 17:13 +0200, Arnau Mir Torres escreveu:
> Hello.
>
> I need to know how can R compute AIC when I study a regression model?
> For example, if I use these data:
> growth tannin
> 1 12 0
> 2 10 1
> 3 8 2
> 4 11 3
> 5 6 4
> 6 7 5
> 7 2 6
> 8 3 7
> 9 3 8
> and I do
> model <- lm (growth ~ tannin)
> AIC(model)
>
> R responses:
> 38.75990
>
> I know the following formula to compute AIC:
> AIC= -2*log-likelihood + 2*(p+1)
>
> In my example, it would be:
> AIC=-2*log-likelihood + 2*2
> but I don't know how R computes log-likelihood:
>
> logLik(model)
> 'log Lik.' -16.37995 (df=3)
Arnau,
LogLik= -16.37995
AIC= -2*log-likelihood + 2*(p+1)
AIC=-2*-16.37995 + 2*(p+1)
AIC= 32.7599+2*(p+1)
#
# this is very important the model have two
# parameter, because sigma is a parameter to.
# so
#
AIC= 32.7599+2*(2+1)
AIC= 32.7599+6
AIC= 38.7599
--
Bernardo Rangel Tura, M.D,MPH,Ph.D
National Institute of Cardiology
Brazil
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