simple interactions
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 differences). 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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