Abstract
This paper proposes a new control chart to effectively monitor the decrease in variance of a normal process when only individual observations are available. The idea of the new control chart, which is called the exponentially weighted mean squared inverse error (EWMSI) chart, stems from the exponentially weighted mean squared error (EWMS) chart of MacGregor and Harris (J. Qual. Technol. 1993; 25(2):106-118). Simulation studies and an example demonstrate that the lower-sided EWMSI chart is superior to the lower-sided EWMS chart and the lower-sided modified EWMS (M-EWMS) chart of Huwang et al. (Nav. Res. Logist. 2009; 56(7):659-668) in detecting a decrease in process variance. Furthermore, the combination of a lower-sided EWMSI chart and an upper-sided EWMS chart is shown to outperform the two-sided EWMS chart and the two-sided M-EWMS chart in monitoring the overall changes in process variance. Copyright © 2010 John Wiley & Sons, Ltd.