Hi Kevin, John, Gabor, and Rolf,
 
Thank you all for your help with my question. Kevin's answer was particularly detailed. It does appear that the code he supplied tests essentially the same model in R as was being tested in SAS.
 
Now I have some a follow-up question about how the degrees of freedom are apportioned in the analysis that I hope people won't mind anaswering. The various degrees of freedom seem to be determined as follows:
 
Patient: N-g = 80-3=77
Treatment: g-1 = 3-1 = 2
Month: t-1 = 6-1 = 5
Treatment*Month: (g-1)(t-1) = (2)(5) = 10
Total: Nt - missing obs = (80)(6)-26 =454
Residual: total - predictors = 454 - 18 = 436
 
For some reason, the denominator df in the SAS analysis are 77 for Treat, 359 for Month, and 359 for Treatment*Month and I did get the same thing in some of my analyses using nlme. If you add 77 and 359, you get the residual df of 436. The result from the gls code just says that the denominator df is 436. So why the difference here? What is the rationale for calculating the denominator df in the various analyses?
 
Thanks,
 
Paul
 
 
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