R-beta: a suggestion for constraints

One small thing that would be very nice for baseline-constraint (R
calls them treatment for some reason) factor variables would be an
option to specify which category is baseline. This would be extremely
useful in experimental contexts, especially medicine. For the moment,

I actually agree here. There seem to be two possibilities. One is to make
it easy to use as.factor() to create factors with code 1 being whatever
level you want. The other is to make it easy to use contrasts by providing
a contr.treatment(baseline) function that creates treatment contrasts with
the specified baseline level, so you could do
glm(outcome~trt,contrasts=list(trt=contr.treatment("PL"))

The first seems a more natural solution to this specific problem, as we
have a natural baseline category and the problem is that as.factor doesn't
know that. In fact the function to change is factor(), which does all the
work of factor creation.  I have a draft version of factor(,baseline=)
but I would like to test it some more (and see if other people think it is
a good idea).  I would personally prefer to call the argument 'reference'
rather than 'baseline' but I don't really care all that much.

BTW, R calls it "contr.treatment" because S does. You could easily define
an alias "contr.whatever.you.like" for it using the .Alias() function.

	-thomas


Thomas Lumley
Asst Professor, Biostatistics, Box 357232
University of Washington, Seattle, WA 98195-7232








-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html
Send "info", "help", or "[un]subscribe"
(in the "body", not the subject !)  To: r-help-request at stat.math.ethz.ch
_._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._