Economic design of Xbar control charts with continuously variable sampling intervals

In the context of economic design, a two-stage model is proposed that utilizes continuously variable sampling intervals. Specifically, each successive sampling interval is determined by the extremity of the latest sample. Modeling the situation as a Markov chain, the hourly cost is developed for any arbitrary set of design parameters. This proposed approach is found to be more economically desirable, possessing a smaller average out-of-control production time, when compared with a standard two-stage approach where the sampling interval alternates between two fixed values. Copyright © 2009 John Wiley & Sons, Ltd.