[Rd] contrasts

Prof Brian Ripley ripley at stats.ox.ac.uk
Wed May 7 17:12:14 MEST 2003

On Wed, 7 May 2003, Torsten Hothorn wrote:

> I spent some time now trying to understand how functions for the
> computation of contrasts
> contr.foo(n, contrasts=TRUE)
> passed to model.matrix are called. My problem is
> that for the computation of some contrasts one needs `n' to be the number
> of observations at each level of the factor of interest and for others the
> number of levels is sufficient. For example, `contr.treatment' has code
> for handling vector valued `n' (and ?contrasts states that `n' may be a vector).
> Where can I find the condition under which `n = nlevel(x)' or `n =
> table(x)' or how can I define what I would like to see?

>From model.matrix, only the number of levels is passed: src/main/model.c 
line 1626 (in R-devel and probably also 1.7.0).

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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