Split Plot Designs

When the levels of some treatment factors are more difficult to change during the experiment than those of others, split-plot designs are necessary. In a split-plot design, the experimental units are called split plots, and are nested within whole plots, which themselves may or may not be nested within blocks. The split plots within each whole plot are assigned at random to the levels of one or more of the treatment factors. The levels of other treatment factors are assigned to whole plots and remain constant for all split plots within a whole plot. A typical model for the split-plot type of designs and the associated analysis are introduced in this chapter through examples. Designs with an extra level of nesting (split-split-plot designs) are briefly described as well. The concepts introduced in this chapter are illustrated through the use of SAS and R software.