Gabor Grothendieck wrote:
> R version 2.10.1 (2009-12-14)
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> ISBN 3-900051-07-0
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> Below are two cases where I don't seem to be getting contr.sum
> contrasts even though they were specified. Are these bugs?
>
> The first case is an interaction between continuous and factor variables.
>
> The second case contrasts= was specified as an arg to lm. The second
> works ok if we set the contrasts through options but not if we set it
> through an lm argument.
In case #2, I think you just failed to read the help page.
> model.matrix(~fac, contrasts=list(fac="contr.sum"))
(Intercept) fac1 fac2
1 1 1 0
2 1 1 0
3 1 1 0
4 1 1 0
5 1 1 0
6 1 0 1
7 1 0 1
8 1 0 1
9 1 0 1
10 1 0 1
11 1 -1 -1
12 1 -1 -1
13 1 -1 -1
14 1 -1 -1
15 1 -1 -1
attr(,"assign")
[1] 0 1 1
attr(,"contrasts")
attr(,"contrasts")$fac
[1] "contr.sum"
As for case #1, the rules are tricky in cases where interactions are
present without main effects, but AFAICS, what you observe is
essentially the same effect as
> model.matrix(~fac-1, contrasts=list(fac="contr.sum"))
fac1 fac2 fac3
1 1 0 0
2 1 0 0
3 1 0 0
4 1 0 0
5 1 0 0
6 0 1 0
7 0 1 0
8 0 1 0
9 0 1 0
10 0 1 0
11 0 0 1
12 0 0 1
13 0 0 1
14 0 0 1
15 0 0 1
attr(,"assign")
[1] 1 1 1
attr(,"contrasts")
attr(,"contrasts")$fac
[1] "contr.sum"
I.e., that R reverts to using indicator variables when the intercept is
absent.
>
>> # 1. In this case I don't seem to be getting contr.sum contrasts:
>>
>> options(contrasts = c("contr.sum", "contr.poly"))
>> getOption("contrasts")
> [1] "contr.sum" "contr.poly"
>> scores <- rep(seq(-2, 2), 3); scores
> [1] -2 -1 0 1 2 -2 -1 0 1 2 -2 -1 0 1 2
>> fac <- gl(3, 5); fac
> [1] 1 1 1 1 1 2 2 2 2 2 3 3 3 3 3
> Levels: 1 2 3
>> # I get this:
>> model.matrix(~ scores:fac)
> (Intercept) scores:fac1 scores:fac2 scores:fac3
> 1 1 -2 0 0
> 2 1 -1 0 0
> 3 1 0 0 0
> 4 1 1 0 0
> 5 1 2 0 0
> 6 1 0 -2 0
> 7 1 0 -1 0
> 8 1 0 0 0
> 9 1 0 1 0
> 10 1 0 2 0
> 11 1 0 0 -2
> 12 1 0 0 -1
> 13 1 0 0 0
> 14 1 0 0 1
> 15 1 0 0 2
> attr(,"assign")
> [1] 0 1 1 1
> attr(,"contrasts")
> attr(,"contrasts")$fac
> [1] "contr.sum"
>
>> # But I was expecting this since I am using contr.sum
>> cbind(1, model.matrix(~ fac)[,2:3] * scores)
> fac1 fac2
> 1 1 -2 0
> 2 1 -1 0
> 3 1 0 0
> 4 1 1 0
> 5 1 2 0
> 6 1 0 -2
> 7 1 0 -1
> 8 1 0 0
> 9 1 0 1
> 10 1 0 2
> 11 1 2 2
> 12 1 1 1
> 13 1 0 0
> 14 1 -1 -1
> 15 1 -2 -2
>>
>> # 2.
>> # here I don't get contr.sum but rather get contr.treatment
>> options(contrasts = c("contr.treatment", "contr.poly"))
>> getOption("contrasts")
> [1] "contr.treatment" "contr.poly"
>> model.matrix(lm(seq(15) ~ fac, contrasts = c("contr.sum", "contr.poly")))
> (Intercept) fac2 fac3
> 1 1 0 0
> 2 1 0 0
> 3 1 0 0
> 4 1 0 0
> 5 1 0 0
> 6 1 1 0
> 7 1 1 0
> 8 1 1 0
> 9 1 1 0
> 10 1 1 0
> 11 1 0 1
> 12 1 0 1
> 13 1 0 1
> 14 1 0 1
> 15 1 0 1
> attr(,"assign")
> [1] 0 1 1
> attr(,"contrasts")
> attr(,"contrasts")$fac
> [1] "contr.treatment"
>
>> model.matrix(lm(seq(15) ~ fac)) # same
> (Intercept) fac2 fac3
> 1 1 0 0
> 2 1 0 0
> 3 1 0 0
> 4 1 0 0
> 5 1 0 0
> 6 1 1 0
> 7 1 1 0
> 8 1 1 0
> 9 1 1 0
> 10 1 1 0
> 11 1 0 1
> 12 1 0 1
> 13 1 0 1
> 14 1 0 1
> 15 1 0 1
> attr(,"assign")
> [1] 0 1 1
> attr(,"contrasts")
> attr(,"contrasts")$fac
> [1] "contr.treatment"
>
>> # I was expecting the first one to give me this
>> options(contrasts = c("contr.sum", "contr.poly"))
>> model.matrix(lm(seq(15) ~ fac))
> (Intercept) fac1 fac2
> 1 1 1 0
> 2 1 1 0
> 3 1 1 0
> 4 1 1 0
> 5 1 1 0
> 6 1 0 1
> 7 1 0 1
> 8 1 0 1
> 9 1 0 1
> 10 1 0 1
> 11 1 -1 -1
> 12 1 -1 -1
> 13 1 -1 -1
> 14 1 -1 -1
> 15 1 -1 -1
> attr(,"assign")
> [1] 0 1 1
> attr(,"contrasts")
> attr(,"contrasts")$fac
> [1] "contr.sum"
>
>> R.version.string
> [1] "R version 2.10.1 (2009-12-14)"
>> win.version()
> [1] "Windows Vista (build 6002) Service Pack 2"
>
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--
Peter Dalgaard
Center for Statistics, Copenhagen Business School
Phone: (+45)38153501
Email: pd....@cbs.dk Priv: pda...@gmail.com
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