Design Issues in Fractional Factorial Split-Plot Experiments

It is often impractical to perform the experimental runs of a fractional factorial in a completely random order. In these cases, restrictions on the randomization of the experimental trials are imposed and the design is said to have a split-plot structure. Similar to fractional factorials, the “goodness” of fractional factorial split-plot designs can be judged using the minimum aberration criterion. However, the split-plot nature of the design implies that not all factorial effects can be estimated with the same precision. In this paper, we discuss the impact of the randomization restrictions on the design. We show how the split-plot structure affects estimation, precision, and the use of resources. We demonstrate how these issues affect design selection in a real industrial experiment.