design matrix creation in R

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Hello,

I want to create a design matrix using R. Can you explain the code  
which creates the following please? I understand the first part.
What first part if the next part is ... this ?
b=g1(?) does what?
That's just generating a factor variable.
dd <- data.frame(a = gl(3,4), b = gl(4,1,12)) # balanced 2-way
dd
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

I am using the tree dataset in R. I want to form a reparameterized  
design matrix in ones, zeroes and minus ones. The dataframe dd is  
very important here.

Can anyone assist here? Thanks in advance.
Something like this?

model.matrix(~a+b, data= dd, contrasts = list(a="contr.sum",  
b="contr.sum") )

    (Intercept) a1 a2 b1 b2 b3
1            1  1  0  1  0  0
2            1  1  0  0  1  0
3            1  1  0  0  0  1
4            1  1  0 -1 -1 -1
5            1  0  1  1  0  0
6            1  0  1  0  1  0
7            1  0  1  0  0  1
8            1  0  1 -1 -1 -1
9            1 -1 -1  1  0  0
10           1 -1 -1  0  1  0
11           1 -1 -1  0  0  1
12           1 -1 -1 -1 -1 -1
attr(,"assign")
[1] 0 1 1 2 2 2
attr(,"contrasts")
attr(,"contrasts")$a
[1] "contr.sum"

attr(,"contrasts")$b
[1] "contr.sum"
David Winsemius, MD
West Hartford, CT
Hello,

I want to create a design matrix using R. Can you explain the code which creates the following please? I understand the first part.
b=g1(?) does what?
That is gl, not g1 (i.e. gee ell not gee one).    See ?gl for a 
description.

Duncan Murdoch
dd <- data.frame(a = gl(3,4), b = gl(4,1,12)) # balanced 2-way
dd
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

I am using the tree dataset in R. I want to form a reparameterized design matrix in ones, zeroes and minus ones. The dataframe dd is very important here.

Can anyone assist here? Thanks in advance.

Mary A. Marion

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