[R] Testing for parallel slopes

[Thomas Lumley]

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 
  options("contrasts")
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.

	-thomas

Thomas Lumley
Assistant Professor, Biostatistics
University of Washington, Seattle

-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html
Send "info", "help", or "[un]subscribe"
(in the "body", not the subject !)  To: r-help-request at stat.math.ethz.ch
_._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._