I am analyzing a set of data from a split-plot experiement using SAS's proc mixed. The resultant "covariance parameter estimate" for the whole plot error term is 0. Someone told me that SAS suggests removing the term from the model. With ddfm=satterth or with the term removed I get the same results.

In both cases the denominator degrees of freedom for testing the whole plot factor is the same as that used to test the split-plot factor. This is inconsistent with my understanding of such things as I understnad them from Kempthorne's 1977 article "Why Randomize". I'm more inclinded to use ddf=n1,n2,n2 where n1 is the number of degrees of freedom that one would classically use i.e., (b-1) * (t -1) where b is the number of blocks and t is the number of main plot factor levels. Here n2 is the denominator degrees of freedom for testing the split-plot factor.

Can someone help me understand an argument for using the suggested method rather that the one I used? Is my approach erroneous? Thanks in advance.

Bert Bishop