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st: Interactions using xtlogit


From   "Sturrock, Hugh" <SturrockH@globalhealth.ucsf.edu>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: Interactions using xtlogit
Date   Wed, 9 May 2012 22:33:01 +0000

Hi,

I am running a random effects model to model infection data (IsPos) using the xtlogit command and want to include an interaction between 2 categorical variables, one with 3 groups (Dist) and the other with 2 groups (Local). I'd ideally like a single p-value to tell me whether the association between IsPos and Dist is modified by Local.

Assuming ID is the household level ID I'm clustering by, if I run the following 

xi:xtlogit IsPos i.Dist*i.Local, i(ID) or

I get two interaction terms DistxLoc_2_2 and DistxLoc_3_2. To see if there is an overall interaction occurring I've used the command

test DistxLoc_2_2 DistxLoc_3_2

My question is, is this the correct approach? Also, when do you use # instead of * to indicate an interaction? Also, if I wanted to make some other pairwise comparisons between interaction groups (i.e. DistxLoc_1_1 with DistxLoc_1_2) how would I do it?

If anyone can comment I would be hugely grateful!
Thanks, Hugh

Dr Hugh Sturrock, PhD
Postdoctoral Fellow
Malaria Elimination Initiative
Global Health Group
University of California, San Francisco
50 Beale Street, Suite 1200, Box 1224
San Francisco, CA 94105 USA
Tel: (+1) 415 597 4669



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