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Re: st: interaction without main effects
typing i.agegrp|durastay (without "i."before durastay) should give
you main effects for durastay plus the interactions with agegrp, but
no main effects for agegroup.
I suppose you read the right thing in help, but got an error by
translating it to your varnames.
> Dear all,
> I might just look at the wrong place, but...
> I'm performing a poisson regression like:
> xi: poisson patient i.origin i.age i.durstay, exp(ptime)
> with 7 agegroups and 9 groups for durstay.
> I want to add an interaction between age and durstay, defined as
> i.agegrp*i.durastay, for which I reallocated agegroups and durstay
> groups to broader categories 3*3. Now of course, the only extra output
> I'd like to see is the interactions, not the main effects.
> The only thing I could find in "help" is:
> i.varname1|varname3 creates dummies for categorical variable
> varname1 and includes continuous variable varname3: all interactions
> and main effect of varname3, but not main effect of varname1.
> So I conclude from that, that if I would do i.agegrp|i.durastay, I would
> get the interactions and the main effect of durastay? That doesn't
> really help, so what character should be used for only interaction and
> no main effect? the FAQ's and websearch didn't really guide me to the
> right direction too.
> I use stata 7
> Thanks for any help!
> A.M. Vos
> dept. of Public Health
> Erasmus MC
> University Medical Center Rotterdam
> P.O. Box 1738
> 3000 DR Rotterdam
> The Netherlands
> Tel: +31 (10) 4087502
> Fax: +31 (10) 4089449
> email: firstname.lastname@example.org
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