[R] about model.matrix

[Michael Dewey]

This is really a question about statistics rather than R but see below

On 01/04/2015 06:28, Fix Ace wrote:
> 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
>

That got mangled but

In your matrix below try forming the sum of a1+a2+a3 and the sum of 
b1+b2+b3+b4. I think you will find they are linearly related.

> 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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>
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-- 
Michael
http://www.dewey.myzen.co.uk/home.html