Re-Post: Combining Factors in model.matrix
On Sat, 24 Jan 2004 02:31:19 -0500, you wrote:
Hello, I didn't get any response on this before, leading me to believe I've missed something fundamental. Can anybody guide me in the correct direction for more help on this?
I didn't see the earlier posting, but nothing seems to be wrong with this one.
I want to be able to create a design matrix with two factors. For instance, if I have:
t1 <- factor(c(1,1,2,2)); t2 <- factor(c(1,2,1,2)); design <- model.matrix(~ -1 + (t1+t2)); design;
t11 t12 t22 1 1 0 0 2 1 0 1 3 0 1 0 4 0 1 1 But the design matrix I want is: t1 t2 1 1 0 2 1 1 3 0 0 4 0 1
You seem to want something like
model.matrix(~ t1+t2 )[,-1]
t12 t22 1 0 0 2 0 1 3 1 0 4 1 1 (i.e. leave the intercept in the model, but delete it from the result). This doesn't give the exact encoding you asked for; the "contrasts" options might be able to fix this (see ?contr.poly, or maybe ?contrasts, and experiment a bit.)
Actually, in general I'm struggling with the syntax for formulating a design matrix I can write down on paper. Is there a reference for this beyond the R documentation?
I don't know. Duncan Murdoch