Split-plot model with covariate
Consider a split-plot model with one whole-plot factor, one sub-plot factor, and a covariate that does not depend on the subplot factor. We discuss the following three key areas where errors in the analysis of this simple model may occur: (1) correct specification of the model, (2) parameterization of variance components, and (3)the final analysis using PROC MIXED (SASv8.2). A further complication arises in cases where the within-whole-plot variability is larger than the between-whole-plot variability. We suggest two options, based on different assumptions, to accommodate cases like these, and the user is to decide which is most appropriate.
Aldworth, W., & Hoffman, WP. (2002). Split-plot model with covariate. American Statistician, 56(4), 284-289. https://doi.org/10.1198/000313002597