A Graphical Approach for Assessing Optimal Operating Conditions in Robust Design

Abstract The determination of optimal operating conditions in robust parameter design often involves the use of an objective function such as the mean squared error of the response along with graphical overlays of the estimated process mean and variance functions. Existing graphical methods in robust design have limitations when more than two control factors are involved, since visualizing the entire control design space becomes difficult with many control factors. Here we present a new graphical technique for assessing optimal operating conditions which is based upon the estimated proportion of items within specification limits. The new plots offer users the ability to compare competing sets of optimal operating conditions, to assess possible assumption violations regarding the distribution of the noise factors, and the opportunity to observe the shape of the response distribution as it relates to stated specification limits. An example from manufacturing serves as the basis for illustration.