Adaptive EWMA control charts with time-varying smoothing parameter

Abstract

Time-weighted charts like EWMA or CUSUM are designed to be optimal to detect a specific shift. This feature, however, can make the chart suboptimal for some other shifts.If, for instance, the charts are designed to detect a small shift, then, they can be inefficient to detect moderate or large shifts. In the literature, several alternatives have been proposed to circumvent this limitation, like the use of control charts with variable parameters or adaptive control charts. This paper aims to propose new adaptive EWMA control charts (AEWMA) based on the assessment of a potential misadjustment, which is translated into a time-varying smoothing parameter. The resulting control charts can be seen as a smooth combination between Shewhart and EWMA control charts, which could be efficient for a wide range of shifts. Markov chain procedures are established to analyse and design the proposed charts. Comparisons with other adaptive and traditional control charts show the advantages of our proposals.