Bugs? when dealing with contrasts
On Wed, Apr 21, 2010 at 4:26 PM, Peter Dalgaard <pdalgd at gmail.com> wrote:
As for case #1, the rules are tricky in cases where interactions are present without main effects, but AFAICS, what you observe is essentially the same effect as
model.matrix(~fac-1, contrasts=list(fac="contr.sum"))
? fac1 fac2 fac3 1 ? ? 1 ? ?0 ? ?0 2 ? ? 1 ? ?0 ? ?0 3 ? ? 1 ? ?0 ? ?0 4 ? ? 1 ? ?0 ? ?0 5 ? ? 1 ? ?0 ? ?0 6 ? ? 0 ? ?1 ? ?0 7 ? ? 0 ? ?1 ? ?0 8 ? ? 0 ? ?1 ? ?0 9 ? ? 0 ? ?1 ? ?0 10 ? ?0 ? ?1 ? ?0 11 ? ?0 ? ?0 ? ?1 12 ? ?0 ? ?0 ? ?1 13 ? ?0 ? ?0 ? ?1 14 ? ?0 ? ?0 ? ?1 15 ? ?0 ? ?0 ? ?1 attr(,"assign") [1] 1 1 1 attr(,"contrasts") attr(,"contrasts")$fac [1] "contr.sum" I.e., that R reverts to using indicator variables when the intercept is absent.
Is there any nice way of getting contr.sum coding for the interaction as opposed to the ugly code in my post that I used to force it? i.e. cbind(1, model.matrix(~ fac)[,2:3] * scores)