On auxiliary information based improved EWMA median control charts

Abstract

Process monitoring is a continuous process for improving the quality. Control chart is a process monitoring tool of SPC tool kit that plays an important role in providing widespread monitoring, to observe the changes in parameters. Mostly, the mean control charts are used for monitoring of process location. In a perfect situation, when there are no outliers, the mean charts are more efficient than median control charts. In reality, data is not always free from outliers, so the median charts are considered as the best for monitoring location parameters. The use of an auxiliary variable in a control chart may be the cause of efficiency gain. The current article considers EWMA median charts based on auxiliary variable(s). Different run length performance measures are considered to expedite the proposed charts in both contaminated and uncontaminated process environments under multivariate normal distributions. An illustrative example is provided to validate the performance of the proposed charts. From the results, we deduce that the performance of median control charts is much better than that of mean control charts in the presence of outliers; moreover, the performance of control charts can be enhanced by using more auxiliary variables.