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st: new to xtmixed - basic question


From   Ricardo Ovaldia <ovaldia@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: new to xtmixed - basic question
Date   Wed, 20 Jul 2011 05:33:01 -0700 (PDT)

Hi,

I am new to  –xtmixed-. 
I am using it to analyze longitudinal data on about 300 patients. The patients were randomized to three treatment groups and a blood metabolite was measured over time. The main interest is the effect of treatment on the metabolite, adjusted for both fixed covariates such as gender and time- changing covariates such as weight.
The model is of the form:
 xtmixed met treatment x1 x2 x3 || participantid:, covar(unstructured)

I have two questions.
1.	How does –xtmixed- know what is the time variable (there are gaps in the data).
2.	When I plot the met over time, if first increases and then decreases forming a upside down “U” or parabola shape. You I include a quadratic term for time in the model. For example:

. gen time2=time^2 

. xtmixed met treatment x1 x2 x3 time time2|| participantid:, covar(unstructured)

Thank you in advance ,
Ricardo



Ricardo Ovaldia, MS
Statistician 
Oklahoma City, OK



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