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st: -xtmixed- and random terms nested within fixed treatment terms?


From   Buzz Burhans <wsb2@cornell.edu>
To   statalist@hsphsun2.harvard.edu
Subject   st: -xtmixed- and random terms nested within fixed treatment terms?
Date   Thu, 16 Jun 2005 11:02:22 -0400

I have a uestion on the new xtmixed. I am trying to match a model run in S-Plux lme, and I am confused by the (lack of) nesting in
-xtmixed-

Suppose I have subjects assigned to treatment with outcomes measured at repeated occasions, and a covariate. It seems that the syntax for -xtmixed- should be something like this, where id(treatment) indicates nesting of id within treatment:

xtmixed outcome covariate treatment occasion || subject(treatment):

I might expect an adaptation of the example for xtmixed on page 184 [XT] to look like this:

xtmixed weight preweight treatment week || id(treatment):

It seems to me that id must be nested in treatment, but treatment is a fixed effect, not random, so the following syntax would be incorrect:

xtmixed weight preweight treatment week || id : treatment

It does not seem possible to nest subject (as a random term) within treatment in -xtmixed-; which seems the appropriate specification. Am I incorrect in thinking that this nesting is necessary?


Thanks
Buzz Burhans


Buzz Burhans
wsb2@cornell.edu


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