[R] Re-Post: Combining Factors in model.matrix

[Prof Brian Ripley]

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