The exponentially weighted moving average (EWMA) control charts with variable sampling intervals (VSIs) have been shown to be substantially quicker than the fixed sampling intervals (FSI) EWMA control charts in detecting process mean shifts. The usual assumption for designing a control
chart is that the data or measurements are normally distributed. However, this assumption may not be true for some processes. In the present paper, the performances of the EWMA and combined X-EWMA control charts with VSIs are evaluated under non-normality. It is shown that adding the VSI feature
to the EWMA control charts results in very substantial decreases in the expected time to detect shifts in process mean under both normality and non-normality. However, the combined X-EWMA chart has its false alarm rate and its detection ability is affected if the process data are not normally
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variable sampling intervals
Document Type: Research Article
Department of Accounting Information, National Taichung Institute of Technology, Taichung, Taiwan, Republic of China
Department of Finance, National Taichung Institute of Technology, Taichung, Taiwan, Republic of China
Publication date: March 1, 2011