Calculating AIC “by hand” in R

Question

I have tried calculating the AIC of a linear regression in R but without using the AIC function, like this:

lm_mtcars <- lm(mpg ~ drat, mtcars)

nrow(mtcars)*(log((sum(lm_mtcars$residuals^2)/nrow(mtcars))))+(length(lm_mtcars$coefficients)*2)
[1] 97.98786

However, AIC gives a different value:

AIC(lm_mtcars)
[1] 190.7999

Could somebody tell me what I'm doing wrong?

Answer 1

Note that the help on the function logLik in R says that for lm models it includes 'all constants' ... so there will be a log(2*pi) in there somewhere, as well as another constant term for the exponent in the likelihood. Also, you can't forget to count the fact that $\sigma^2$ is a parameter.

$\cal L(\hat\mu,\hat\sigma)=(\frac{1}{\sqrt{2\pi s_n^2}})^n\exp({-\frac{1}{2}\sum_i (e_i^2/s_n^2)})$

$-2\log \cal{L} = n\log(2\pi)+n\log{s_n^2}+\sum_i (e_i^2/s_n^2)$

$= n[\log(2\pi)+\log{s_n^2}+1]$

$\text{AIC} = 2p -2\log \cal{L}$

but note that for a model with 1 independent variable, p=3 (the x-coefficient, the constant and $\sigma^2$)

Which means this is how you get their answer:

nrow(mtcars)*(log(2*pi)+1+log((sum(lm_mtcars$residuals^2)/nrow(mtcars))))
       +((length(lm_mtcars$coefficients)+1)*2)

Answer 2

The AIC function gives $2k -2 \log L$, where $L$ is the likelihood & $k$ is the number of estimated parameters (including the intercept, & the variance). You're using $n \log \frac{S_{\mathrm{r}}}{n} + 2(k-1)$, where $S_{\mathrm{r}}$ is the residual sum of squares, & $n$ is the sample size. These formulæ differ by an additive constant; so long as you're using the same formula & looking at differences in AIC between different models where the constants cancel, it doesn't matter.