[R] simple interactions

Achim Zeileis Achim.Zeileis at uibk.ac.at
Fri Apr 15 14:51:31 CEST 2016

Try using
   ~ group/age
or even
   ~ 0 + group/age

Both have all three group-specific slopes but differ with respect to the 
intercept codings. The latter has three group-specific intercepts as well. 
But the former has an intercept corresponding to the reference group A and 
then the usual treatment contrasts for group B and C (i.e., intercept 

IIRC then I found the discussion of these contrasts and nested codings in 
the MASS book very useful.

On Fri, 15 Apr 2016, Therneau, Terry M., Ph.D. wrote:

> I'd like to get interaction terms in a model to be in another form.  Namely, 
> suppose I had variables age and group, the latter a factor with levels A, B, 
> C, with  age * group in the model.  What I would like are the variables 
> "age:group=A", "age:group=B" and "age:group=C"  (and group itself of course). 
> The coefficients of the model will then be the age effect in group A, the age 
> effect in group B and the age effect in C rather than the standard ones of an 
> overall age effect followed by contrasts.  These is often a better format for 
> tables in a publication.
> Yes, I can reconstruct these from the original fit, but I have a lot of 
> variables for several models and it would be easier to have an automatic 
> form.  I suspect that there is an easy answer, but I don't see it.
> Terry Therneau
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