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Analysis of Split‐plot Designs with Mirror Image Pairs as Sub‐plots

In this article we present a procedure to analyze split‐plot experiments with mirror image pairs as sub‐plots when third‐ and higher‐order interactions can be assumed negligible. Although performing a design in a split‐plot manner induces correlation among observations, we show that with such designs the essential search for potentially active factors can be done in two steps using ordinary least squares. The suggested procedure is tested out on a real example and on two simulated screening examples; one with a split‐plot design based on a geometric design and one with a split‐plot design based on a non‐geometric Plackett and Burman design. The examples also illustrate the advantage of using non‐geometric designs where the effects are partially aliased instead of being fully aliased as in highly fractionated fractional factorials. Copyright © 2005 John Wiley & Sons, Ltd.