On Sep 17, 2009, at 11:13 AM, Peng Yu wrote:
Hi,
I don't understand what the meaning of the following lines returned by
model.matrix(). Can somebody help me understand it? What can they be
used for?
attr(,"assign")
[1] 0 1 2 2
attr(,"contrasts")
attr(,"contrasts")$A
[1] "contr.treatment"
attr(,"contrasts")$B
[1] "contr.treatment"
?contrasts.
---direct quote---
contr.treatment contrasts each level with the baseline level
(specified by base): the baseline level is omitted. Note that this
does not produce ‘contrasts’ as defined in the standard theory for
linear models as they are not orthogonal to the intercept.
---end quote---
Also read through (again?):
?aov
--
David.
Regards,
Peng
a=2
b=3
n=4
A = rep(sapply(1:a,function(x){rep(x,n)}),b)
B = as.vector(sapply(sapply(1:b, function(x){rep(x,n)}), function(x)
{rep(x,a)}))
Y = A + B + rnorm(a*b*n)
fr = data.frame(Y=Y,A=as.factor(A),B=as.factor(B))
afit=aov(Y ~ A + B,fr)
model.matrix(afit)
(Intercept) A2 B2 B3
1 1 0 0 0
2 1 0 0 0
3 1 0 0 0
4 1 0 0 0
5 1 1 0 0
6 1 1 0 0
7 1 1 0 0
8 1 1 0 0
9 1 0 1 0
10 1 0 1 0
11 1 0 1 0
12 1 0 1 0
13 1 1 1 0
14 1 1 1 0
15 1 1 1 0
16 1 1 1 0
17 1 0 0 1
18 1 0 0 1
19 1 0 0 1
20 1 0 0 1
21 1 1 0 1
22 1 1 0 1
23 1 1 0 1
24 1 1 0 1
attr(,"assign")
[1] 0 1 2 2
attr(,"contrasts")
attr(,"contrasts")$A
[1] "contr.treatment"
attr(,"contrasts")$B
[1] "contr.treatment"
David Winsemius, MD
Heritage Laboratories
West Hartford, CT
______________________________________________
R-help@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
