[R] Testing for parallel slopes
thomas at biostat.washington.edu
Tue Aug 1 22:46:44 CEST 2000
On Tue, 1 Aug 2000, Marc Feldesman wrote:
> My question has to do with the issue of parametrization. When I use the
> example data set "whiteside", I get the same results that VR get (see p.
> 154) except I'm getting them *without* reparametrizing the model. (In fact,
> I get the same answers with the default parametrization and the
> alternative). Since I'm only interested in the interaction term, is it
> even necessary to change the structure of the model matrix, or will I get
> the answer I need using the default contrast matrices (i.e.
> "contr.helmert", "contr.poly")?
In R "contr.helmert" is *not* the default. Type
to see what contrasts you are using.
This and other differences are described in both the "R Complements" to
V&R3 (see http://www.stats.ox.ac.uk/pub/MASS3) and the R FAQ.
Assistant Professor, Biostatistics
University of Washington, Seattle
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