multiple pairwise slope comparisons

At 06:36 AM 3/10/2002 -0400, John Janmaat wrote:

I have a linear model with different slopes for different treatment 
groups.  I need to pairwise compare the different slope estimates for 
the different treatment groups.  Is there a package that does pairwise 
comparisons of slope coefficients, making the appropriate adjustments 
in the P values?

Dear John,

I'm not aware of anything that does this directly, but there are several 
simple ways of making these comparisons.

For example, if you use treatment contrasts (the default in R) and fit a 
model with a constant, then the coefficients for the interaction will 
give you slope comparisons with the the baseline category (and the 
associated t- and p-values). You could then use the linear.hypothesis 
function in the car package to compare other pairs of slopes (via pairs 
of coefficients for the interaction), or, alternatively, refit the model 
with different baseline categories until you have all of the pairwise 
slope comparisons.

Once you have the p-values for the pairs, you could use p.adjust to get 
adjusted p-values, compute Scheffe tests, or whatever you had in mind.

If this is something that you need to do a lot, it wouldn't be hard to 
write a small function to implement the procedure.

I hope that this helps,
 John