I tried to run the sample code from R:
dd <- data.frame(a = gl(3,4), b = gl(4,1,12)) a b
1 1 1
2 1 2
3 1 3
4 1 4
5 2 1
6 2 2
7 2 3
8 2 4
9 3 1
10 3 2
11 3 3
12 3 4
options("contrasts")
model.matrix(~ a + b, dd)(Intercept) a2 a3 b2 b3 b4
1 1 0 0 0 0 0
2 1 0 0 1 0 0
3 1 0 0 0 1 0
4 1 0 0 0 0 1
5 1 1 0 0 0 0
6 1 1 0 1 0 0
7 1 1 0 0 1 0
8 1 1 0 0 0 1
9 1 0 1 0 0 0
10 1 0 1 1 0 0
11 1 0 1 0 1 0
12 1 0 1 0 0 1
when I tried to remove the intercept from the matrix, I used the following
codemodel.matrix(~ 0+a + b, dd)
a1 a2 a3 b2 b3 b41 1 0 0 0 0 02 1 0 0 1 0 03 1 0 0 0 1 04 1 0 0 0 0 15 0 1 0 0
0 06 0 1 0 1 0 07 0 1 0 0 1 08 0 1 0 0 0 19 0 0 1 0 0 010 0 0 1 1 0 011 0 0 1
0 1 012 0 0 1 0 0 1 when I tried to remove the intercept
Here I noticed that, all levels of a, a1, a2, and a3, were included. I wonder
how I can include the "b1" in the matrix as well? a1 a2 a3 b1 b2 b3 b4
1 1 0 0 1 0 0 0
2 1 0 0 0 1 0 0
3 1 0 0 0 0 1 0
4 1 0 0 0 0 0 1
5 0 1 0 1 0 0 0
6 0 1 0 0 1 0 0
7 0 1 0 0 0 1 0
8 0 1 0 0 0 0 1
9 0 0 1 1 0 0 0
10 0 0 1 0 1 0 0
11 0 0 1 0 0 1 0
12 0 0 1 0 0 0 1
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