[R] polynomial contrasts in R

ripley@stats.ox.ac.uk ripley at stats.ox.ac.uk
Fri Jan 17 09:00:04 CET 2003

You are wrong about S-PLUS!  contr.poly only takes as argument the number 
of levels required (and uses pre-computed sets for up to 12 levels).
You get orthogonal polynomials for equally-space values from contr.poly in 
both systems, as documented.

I don't think you are describing contrasts for an ordered factor, but 
orthogonal polynomials in a numeric variable.  The latter are computed by 
the function poly() in both R and S-PLUS.  You could set them up to give a 
contrasts matrix if you want, but not a contrasts function (as that is 
only passed the number of levels, AFAIR).

On Thu, 16 Jan 2003, George W. Gilchrist wrote:

> In S-Plus, I can obtain polynomial contrasts for an ordered factor with
> contr.poly(). The function also exists in R, however is limited to factors
> where the levels are equally spaced. In S-Plus, one can obtain the contrasts
> for a set of numeric values representing unequally spaced ordered factors.
> Has anyone implemented this in R? I see that the S-Plus function calls
> another function (poly.raw()) that calls a Fortran routine. Thanks for your
> assistance.

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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