[R] testing for significantly different slopes

[Dylan Beaudette]

Hi,

How would one go about determining if the slope terms from an analysis of 
covariance model are different from eachother?

Based on the example from MASS:

library(MASS)

# parallel slope model
l.para <- lm(Temp ~ Gas + Insul, data=whiteside)

# multiple slope model
l.mult <- lm(Temp ~ Insul/Gas -1, data=whiteside)

# compare nested models:
anova(l.para, l.mult)
Analysis of Variance Table

Model 1: Temp ~ Gas + Insul
Model 2: Temp ~ Insul/Gas - 1
  Res.Df    RSS Df Sum of Sq      F  Pr(>F)  
1     53 52.045                              
2     52 48.704  1     3.341 3.5673 0.06451 .

It seems like this approach would yield insight into whether or not the two 
slope terms (InsulBefore:Gas  and InsulAfter:Gas) were different. However, is 
there a formal test for this sort of question, and can it be generalized to 
differences between more than 2 slope terms?

Thanks,

Dylan


-- 
Dylan Beaudette
Soil Resource Laboratory
http://casoilresource.lawr.ucdavis.edu/
University of California at Davis
530.754.7341