As it turns out, my split plot design has imbalanced groups. I was able to adequately address this imbalance by moving from 'containment' for the degrees of freedom to Fai-Cornelius (ddfm=satterth). I found this solution in Schaalje et. al. Approximations to Distributions of Test Statistics in Complex Mixed linear Models using SAS PROC MIXED. (see attached, page 2) This solution was a radical improvement to the processing resource demands. This went from taking over 15 minutes to process 5% of my sample to running the entire sample in under 2 minutes. This was extremely helpful for my context as I am using an enumerative iteration process in the macro and running 70 iterations across 6 conditions i.e. I needed to run (now complete) the the entire sample 420 times. I also believe the results are more accurate using this new method. Any further thoughts or feedback are most welcome!
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