The performance of a control chart is commonly investigated based on the assumption of known process parameters. Nevertheless, in most manufacturing and service applications, the process parameters are usually unknown to practitioners. Hence, they are estimated from an in-control Phase-I samples. As such, the performance of the control chart with estimated process parameters will behave differently from the corresponding chart with known process parameters. To study this issue, the exponentially weighted moving average (EWMA) median chart is examined in this article. The EWMA median chart is traditionally investigated based on the average run length (ARL). The limitation of the ARL is that it requires practitioners to specify the shift size in advance. This phenomenon is not ideal for practitioners who do not have background knowledge of the process. In view of this, the EWMA median chart with known and estimated process parameters is studied based on the ARL and expected average run length (EARL). The results indicate that as long as the particular shift size is within the range of shifts, the performance of the chart is almost the same, for the EWMA median chart with known and estimated process parameters.
A control chart is a crucial statistical tool that is typically used to monitor a process, inan effort to reduce variation. It is necessary to practically evaluate the overall performanceof a control chart. Evaluation reveals the effciency of a control chart to detect a processshift. This influences practitioners' decisions on the choice of a control chart. The usualpractice taken to evaluate the performance of a control chart is to use the average runlength (ARL). The assumption for using the ARL as a performance measure is that theshift size is known in advance. Determining the shift size can be restrictive, particularlyin the case practitioners do not have prior knowledge about the process. In view of this,the expected ARL (EARL) is employed as a performance measure for the random processmean shift. In this article, the overall performance of the side sensitive group runs (SSGR)chart is investigated in terms of EARL. Moreover, the optimal design for the SSGR chartbased on minimising the EARL is proposed.
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