[R] Testing for parallel slopes

Marc Feldesman feldesmanm at pdx.edu
Tue Aug 1 22:34:55 CEST 2000

I'm running a series of simple bivariate linear regressions on grouped 
data.  I want to test the slopes to see if they are parallel.  I normally 
use analysis of covariance to do so, looking at interaction between the 
covariate and the factor to make this determination.

VR3 pp.149 - 154 has a very nice example of an ANOCOVA, ending with a 
discussion of this very operation.

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")?

Dr. Marc R. Feldesman
Professor and Chairman
Anthropology Department
Portland State University
1721 SW Broadway
Portland, Oregon 97201
email:  feldesmanm at pdx.edu
phone:  503-725-3081
fax:    503-725-3905

"Anyway, no drug, not even alcohol, causes the fundamental ills of society.
If we're looking for the source of our troubles, we shouldn't test people
for drugs, we should test them for stupidity, ignorance, greed and love of
power."   P.J. O'Rourke

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