On Sun, 1 Aug 2004, Doran, Harold wrote:
> Is there an efficient way to create a series of dummy codes from a
> single variable? For example, I have a variable, “grade” = {2, …,
> 12}. I want to create k-1 dummy codes for grade such that grade 2 is the
> base (i.e, grade 2 =0).
Yes, and that's what R does by default.
> I am hoping that the new variables can be labeled as grade.3, grade.4
> etc. I'll then use
>
> grade <- paste("grade.", 3:12, sep="") in as.formula to build the model.
Why? Just set the levels of your factor to 2:12 and R's default treatment
contrasts do this for you. To see this, just call model.matrix, as in
grade. <- as.factor(sample(2:12, 100, replace=T))
model.matrix(~grade.)[, -1]
but the R model fitting functions will do that for you.
--
Brian D. Ripley, [EMAIL PROTECTED]
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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