Dear list
 
In HLM, one can specify a covariate at one of the "levels". For example, if the data structure are repeated observations nested within students nested within schools, school size might be a covariate that is used at level 3, but not at the other levels. In HLM this is rather easy to do. 
 
However, how can one specify a covariate in R for only one of the levels? I have a sample data set with the structure as described above. I fit the unconditional model in R as
 
model1<-lme(math~year, random=~year|schoolid/childid, data=datafile)
 
Now, if I want to enter "female" as a covariate at level 2 only, how might I modify the code to accomplish this? 
 
------
Harold C. Doran
Director of Research and Evaluation
New American Schools
675 N. Washington Street, Suite 220
Alexandria, Virginia 22314
703.647.1628
 <http://www.edperform.net/>  
 
 
 

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