[R] simple interactions
Achim.Zeileis at uibk.ac.at
Fri Apr 15 14:51:31 CEST 2016
~ 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
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
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> and provide commented, minimal, self-contained, reproducible code.
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