[R] simple interactions

Thierry Onkelinx thierry.onkelinx at inbo.be
Fri Apr 15 14:15:40 CEST 2016

Dear Terry,

Does fitting group + age:group instead of age*group solves your problem?

Best regards,

ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek / Research Institute for Nature and
team Biometrie & Kwaliteitszorg / team Biometrics & Quality Assurance
Kliniekstraat 25
1070 Anderlecht

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2016-04-15 13:58 GMT+02:00 Therneau, Terry M., Ph.D. <therneau op mayo.edu>:

> 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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