Re-Post: Combining Factors in model.matrix

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

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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