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st: anova & repeated measures


From   <cthompson@dfpm.utah.edu>
To   statalist@hsphsun2.harvard.edu
Subject   st: anova & repeated measures
Date   Wed, 07 Apr 2004 18:50:52 -0600

Hello All ---
I am using Intercooled Stata 8.2.
I have a large dataset that contains 2,388 unique subjects where 
each subject has a measurement (or missing value) at 8 distinct 
time points & each subject is categorized into one of four drug 
groups.  My objective is to determine if my outcome varies 
w/respect to drug type.  Per the -anova- syntax requirement, I 
reshape the data to long form with a resultant 19,104 
'observations' (2,388 multiplied by 8); among the 19,104 
records, 11,719 do NOT contain missing values for my outcome 
variable (in short:  unbalanced design).  My question concerns 
the execution of the repeated measures and subsequent 
determination of a proper error term.  If I use the following 
code, Stata returns an error (also pasted):
-anova pdc pt cohort1 qtr, repeated(qtr)- 
too many variables or values
where 'pt' is the subject identifier, 'cohort1' is drug type 
(among 4), and 'qtr' is the timepoint (quarter of year: 1-8).  
If I eliminate 'pt' from the syntax & specify -bseunit- with 
'pt' as the argument, the anova executes fine (I think):
-anova pdc cohort1 qtr, repeated(qtr) bseunit(pt)-
The ANOVA output indicates that the residual is a function of 
'pt' (df of residual is 11,708); herein lies my question:  is 
this logical?  And if so, why did the first syntax presented not 
work even when I maximized matsize?  Note that the df of 
'cohort1' & 'qtr' are logical (3 & 7, respectively).  
Parenthetically, the above ANOVA indicates that my outcome does 
indeed differ across drug type.  I am relatively new to repeated 
measures analysis so I may be overlooking an important 
component/assumption of this technique and if so, can you please 
elaborate or point me in the right direction?  
Any help or suggestions are greatly appreciated!!
Many thanks, Clint Thompson  

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