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Re: st: interaction without main effects


From   "Jonathan Beck" <beck@wz-berlin.de>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: interaction without main effects
Date   Mon, 13 Jan 2003 17:19:11 +0100

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.

Jonathan

> 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! 
> Annelies
> -- 
> 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: vos@mgz.fgg.eur.nl
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