[R] type III Sum Sq in ANOVA table - Howto?
Thomas Lumley
tlumley at u.washington.edu
Fri Mar 7 02:33:03 CET 2003
On Fri, 7 Mar 2003, Josef Frank wrote:
> Hello,
>
> as far as I see, R reports type I sums of squares. I'd like to get R to
> print out type III sums of squares.
>
> e.g. I have the following model:
> vardep~factor1*factor2
>
> to get the type III sum of squares for factor1 I've tried
> anova(lm(vardep~factor2+factor1:factor2),lm(vardep~factor1*factor2))
> but that didn't yield the desired result.
>
> Could anyone give me a hint how to proceed?
>
Unfortunately the arguments about whether Type III sums of squares are
part of the axis of evil have drowned out a real issue.
I would have expected the command to work, and in fact wrote a FAQ answer
saying this was the way to do it. However, if factor1 is indeed a factor
its main effect is helpfully stuck back in the model by terms.formula.
I think this is a bug, since it doesn't happen if factor1 isn't a factor,
and leaving aside any question about Type III SS it seems to make it
impossible to fit the model
lm(vardep~factor2+factor1:factor2)
While this model isn't terribly often useful, it is sometimes.
-thomas
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