Statistically designed experiments are an important tool for the quality and/or reliability engineer. Their role in improvement and optimization of manufacturing processes has been well documented. Designed experiments have been used in industrial process development for well over 50 years, so one might expect that new and important developments are relatively infrequent. However, experimental design is a vibrant field of research. This is being spurred on by applications beyond the traditional manufacturing process arena, and by the discovery that variations of old techniques have new and modern applications.An example of the latter scenario is split-plot designs (SPDs). A SPD involves two kinds of factors, those that are easy to change and those that are hard to change. An example might involve temperature as the hardto-change factor and feed rate as the easy-to-change factor. The SPD employs two levels of randomization to minimize the number of changes of the hard-to-change factor. The hard-to-change factor is assigned to the whole-plot (WP) experimental units, and then the easy-to-change factors are randomly assigned to experimental units within each WP, which are referred to as sub-plots or split-plots (SPs). The SPD has an agricultural heritage, with the WPs often being large plots of ground and the sub-plots being smaller plots of ground within the WPs.As industrial experiments often involve both easy-and hard-to-change factors, there has been widespread renewed interest in the SPD. However, the WP and SP structure of the design leads to two error structures, with two different variance components, one for the WP and one for the SP. Consequently, generalized least squares must be used to fit the model associated with the design. Parker et al. 1 describe classes of SPDs that have an equivalent estimation property; that is, the ordinary least-squares estimates of the model parameters are identical to the generalized least-squares estimates. This property provides the best linear unbiased estimators and simplifies the estimation of the model parameters because the experimenter can use standard computer software that may not have the generalized least-squares capability to fit the underlying models. Their methodology can be applied to many standard response surface designs, including the central composite design, the small composite design, equiradial designs, and the Box-Behnken design. Liang et al. 2 show how fraction of design space (FDS) plots can be extended to evaluate the properties of SPDs. The FDS plot is a graphical representation of the prediction variance from the model that has been fit to the data from the experimental design. It plots the prediction variance (or the scaled prediction variance) versus the fraction of the design space that has prediction variance values at or below a given value. These plots are very valuable in studying the potential performance of a design, and they have been completely randomized designs have been studied extensively with FDS plots and other graphical methods....