Charts With Variable Parameters
Costa, Antonio F.B.
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The idea of varying the chart parameters has been explored extensively in recent years. Basically, the value establishes if the control should be relaxed or not. When falls near the target the control is relaxed because one will wait more to take the next sample and/or the next sample will be smaller than usual. When falls far from the target but not in the action region the control is tightened because one will wait less to take the next sample and/or the next sample will be larger than usual. In this paper, we extend this study to consider the action limits variable too. The idea is to draw the action limits wider than usual when the control is relaxed and narrower than usual when the control is tightened. This new feature makes the chart comparable with the CUSUM and EWMA schemes in terms of the speed they detect small shifts in the process mean.