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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