[R] changing model matrix restriction in lm()

[Bert Gunter]

?contrasts
?C

Cheers,
Bert

On Wed, Oct 23, 2013 at 6:00 AM, Rafael Moral
<rafa_moral2004 at yahoo.com.br> wrote:
> Dear useRs,
> I was wondering if there was a way of changing the model matrix restriction automatically in the formula statement when fitting a model using, for example, lm().
> When we do
>
> set.seed(100)
> y <- rnorm(12)
> A <- gl(3, 4)
> summary(lm(y ~ A))
>
> we obtain A1=0 as a baseline and A2 and A3 as effects.
> However, if I want to use A2=0 as a baseline, I can do
>
> X <- cbind(1, model.matrix(lm(y ~ A - 1)))[,-3]
> summary(lm(y ~ X))
>
> I wonder if there is a way of specifying the restriction in the parameters directly in the formula argument instead of building the desired model matrix.
>
> Best wishes,
> Rafael.
>
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-- 

Bert Gunter
Genentech Nonclinical Biostatistics

(650) 467-7374