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The t and the exponentially weighted moving average (EWMA) t charts were introduced in quality control literature for cases when users are not able to accurately estimate the process standard deviation. In this paper we present the synthetic versions of the t and the EWMA-1 charts, and determine near-optimized control limits for these charts. The new charts are shown to have improved performance properties over the original t and EWMA-t charts, while maintaining the desirable feature that the process standard deviation does not need to be estimated.
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Calzada et al. (2013) studied this question.
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