Economic designs of constrained EWMA and combined EWMA-X¯ control schemes

Abstract

This research presents a comparison between the cost performance of the Exponentially Weighted Moving Average EWMA) and the combined EWMA-X¯ control chart schemes. In particular, we explore the impact of constraining the in-control average run length on the optimal cost performance of both schemes. Methodologically, we incorporate traditional expected cost models and study the robustness of the two approaches. In general, there appears to be minimal motivation to combine the use of both charts within the same application. The cost model for the combined chart is not a well-behaved function, and yields varying optimal parameters when the in-control average run length is constrained.