[R] Re-Post: Combining Factors in model.matrix
Prof Brian Ripley
ripley at stats.ox.ac.uk
Sat Jan 24 13:37:46 CET 2004
On Sat, 24 Jan 2004 paul.boutros at utoronto.ca wrote:
> I didn't get any response on this before, leading me to believe I've missed
> something fundamental. Can anybody guide me in the correct direction for more
> help on this?
You will need to explain to us why the object you list is `the design
matrix': have *you* a reference for that? R is doing the conventional
thing, and I at least have no idea where your example comes from.
You seem to have coded variables t1 and t2 the opposite ways (the
reference level is 2 for t1 and 1 for t2), and your model has the fit at
levels t1=2,t1=1 constrained to pass through the origin. I don't think R
has a simple syntax for that (although you can fake anything), and I find
it hard to believe that is actually what you want.
>
> Paul
>
> =================================================
> I want to be able to create a design matrix with two factors. For instance, if
> I have:
>
> > t1 <- factor(c(1,1,2,2));
> > t2 <- factor(c(1,2,1,2));
> > design <- model.matrix(~ -1 + (t1+t2));
> > design;
> t11 t12 t22
> 1 1 0 0
> 2 1 0 1
> 3 0 1 0
> 4 0 1 1
>
> But the design matrix I want is:
> t1 t2
> 1 1 0
> 2 1 1
> 3 0 0
> 4 0 1
>
> Actually, in general I'm struggling with the syntax for formulating a design
> matrix I can write down on paper. Is there a reference for this beyond the R
> documentation?
Chapter 6 of MASS has the most complete exposition (by Bill Venables) that
I know of, and the White Book (Chambers & Hastie, 1992) goes well beyind
the R documentation (which uses it as the reference).
--
Brian D. Ripley, ripley at stats.ox.ac.uk
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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