[R] Help with contrast settings in glm and lm
Prof Brian Ripley
ripley at stats.ox.ac.uk
Mon Jan 17 13:14:24 CET 2000
> Date: Fri, 14 Jan 2000 12:14:21 +0100 (CET)
> From: Jesús María Frías Celayeta <iosu at ensia.inra.fr>
> I am trying to validate some R code that we have been porting from
> S (ellipse library) and now we are on the step of trying with some
> examples to see if the code gives the same results as in S. I have
> received some results from S-PLUS 4.5 help examples and I have some
> trouble reproducing them. Apparently S+ uses helbert contrasts in
> categorical variables as a default and R uses contrasts based in
> orthogonal polynomials. However, in R there exists contr.helmert and I
> would like to actually set R in a way that a regression procedure will use
> helmert type contrasts (as in S+), in order to have the same results.
> Is there a way to change the default contrasts used in regression
> (lm and glm) so that after it the regression uses those contrasts? I have
> tried options() with no too much sucess. I am probably loosing some page
> in the manual, so just in the case I excuse myself in advance.
but you have been changing the wrong one. The first is for unordered
factors, the second for ordered factors. The R help page does not say so, and
should: I have altered it to give the details and include this example.
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 272860 (secr)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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