On Auxiliary Information Based Improved EWMA Median Control Charts

Document Type: Research Note

Authors

1 School of Mathematical Sciences, Dalian University of Technology Dalian, 116024, P. R. China

2 Department of Mathematics, COMSATS Institute of Information Technology, Wah Cantt 47040, Pakistan

3 Department of Mathematics and Statistics, King Fahad University of Petroleum and Minerals, Dhahran, 31261, Saudi Arabia

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 in process location. In a perfect situation, when there are no outliers, the mean charts are more efficient than median control charts. In reality that data is not free from outliers always, 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 proposed charts. From the results, we deduce that the performance of median control charts is much better than mean control charts in the presence of outliers and also the performance of control charts can be enhanced by using more auxiliary variables.

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