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34048
2009 Jan 23
1
Interpreting model matrix columns when using contr.sum
With the following example using contr.sum for both factors,
> dd <- data.frame(a = gl(3,4), b = gl(4,1,12)) # balanced 2-way
> model.matrix(~ a * b, dd, contrasts = list(a="contr.sum", b="contr.sum"))
(Intercept) a1 a2 b1 b2 b3 a1:b1 a2:b1 a1:b2 a2:b2 a1:b3 a2:b3
1 1 1 0 1 0 0 1 0 0 0 0 0
2 1 1 0 0 1 0