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

[Buzz Burhans <wsb2@cornell.edu>]

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


*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/