[R] AIC v. extractAIC

Ben Bolker bolker at ufl.edu
Fri Dec 7 04:11:25 CET 2007

Ryan Briscoe Runquist wrote:
> Hello,
> I am using a simple linear model and I would like to get an AIC value.  I
> came across both AIC() and extractAIC() and I am not sure which is best to
> use.  I assumed that I should use AIC for a glm and extractAIC() for lm,
> but if I run my model in glm the AIC value is the same if I use AIC() on
> an
> lm object.  What might be going on?  Did I interpret these functions
> incorrectly?
> Thanks,
> Ryan

  The documentation for these functions does explain what's going on,
albeit fairly tersely.  The bottom line is that you should be OK with either
one, provided that you stick to one or the other.

x = runif(100)
y = 1 + 2*x+3*x^2+rnorm(100,sd=0.02)

lm1 = lm(y~x)
lm2 = lm(y~x+I(x^2))

## different

## the same

lm3 = glm(y~x)
lm4 = glm(y~x+I(x^2))

## the same

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