Dear list,

I am confused about two functions in R: AIC(fm) and extractAIC(fm).  What is
the difference between two and when do I have to use one over the other?  I
have found the similar question previously and still not clear for me to
understand.  I also looked at '?AIC' and '?extractAIC' in R, which is also
unclear.  I pasted faked data set, fitting summary, and AICs.

Thank you very much in advance!

Steve


> y <- rnorm(100, 10,3)
> x1 <- y + rnorm(100, 5,3)
> x2 <- y + rnorm(100, 10, 5)
> fm <- lm(y ~ x1+x2)
> summary(fm)

Call:
lm(formula = y ~ x1 + x2)

Residuals:
     Min       1Q   Median       3Q      Max
-4.15818 -1.28600 -0.09738  1.34688  4.61042

Coefficients:
            Estimate Std. Error t value Pr(>|t|)
(Intercept) -0.06431    0.77240  -0.083    0.934
x1           0.43208    0.04334   9.970  < 2e-16 ***
x2           0.16926    0.03357   5.043 2.13e-06 ***
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

Residual standard error: 1.856 on 97 degrees of freedom
Multiple R-squared: 0.6462,     Adjusted R-squared: 0.639
F-statistic:  88.6 on 2 and 97 DF,  p-value: < 2.2e-16

> AIC(fm)
[1] 412.4119
> extractAIC(fm)
[1]   3.0000 126.6242
>

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