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st: Re: Margins and interactions


From   Arti Rayit <arti.rayit@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: Re: Margins and interactions
Date   Fri, 13 May 2011 12:15:02 +0100 (BST)

I have a longitudinal experiment with 3 factors, a (3 levels) b (2 levels) and c 
(2 levels) . As there is missing data and the data is not collected at regular 
time intervals I have used xtmixed to analyse the data:

xtmixed sc c.et##a##b##c II pid:, ml
margins a##b##c, post  asbalanced emptycells(reweight) 

I would like to test if the a#b and possibly other interactions are significant. 
Is it acceptable to fit the model with and without the a#b interaction and 
compare the results with lrtest? I ask this because I understand that the way 
the factors are coded can cause some difficulties in the interpretation of 
effects.

Thank you,

Arti


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