Economic design of dual-sampling-interval policies for (X)over-bar charts with and without run rules

Recent studies show that the dual-sampling-interval (DSI) policies of (X) over bar control chart yield a smaller average time to signal (ATS) than Shewhart's classical fixed-sampling-interval (FSI) policy for off-target processes. An economic design approach for DSI policies has not been addressed in the literature. In this paper we develop a comprehensive cost model for DSI policies with and without run rules and steady-state performance. The expression for the unit cost of quality is used as the objective function in optimal design of DSI policy parameters. The design process and the sensitivities of some of the model input parameters are exposed through numerical examples.