Using a new VSI EWMA average loss control chart to monitor changes in the difference between the process mean and target and/or the process variability

A single chart, instead of X¯ and R   charts or X¯ and S   charts, to simultaneously monitor the process mean and variability would reduce the required time and effort. A number of studies have attempted to find such charts. Moreover, a number of studies demonstrated that the adaptive control charts may detect process shifts faster than the fixed control charts. This paper proposes an easier EWMA average loss chart with variable sampling intervals to effectively monitor the process and diagnose whether the out-of-control process is caused by the changes in the difference of process mean and target or the increase in process variability, or both. Furthermore, an optimal variable sampling interval EWMA average loss chart is also proposed. An example is used to illustrate the application and evaluate the performance of the proposed control chart in detecting and diagnosing the increases in the difference of the process mean and target and/or the process variability. Numerical analyses demonstrated that the optimal VSI EWMA average loss chart outperforms the average loss chart with fixed design parameters and the Shewhart joint X¯ and S charts. Therefore, the (optimal) VSI EWMA average loss chart is recommended.