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EWMA Control Charts for the Smallest and Largest Observations

Summary: [This abstract is based on the authors' abstract.] A proposal is made for an exponentially weighted moving average (EWMA) control chart based on the smallest (Min) and largest (Max) observations in each simple. This MaxMin EWMA chart shows which parameters have increased or decreased when there is a change in the process. The MaxMin EWMA chart also may be viewed as smoothed tolerance limits, and it offers graphical guidance for monitoring processes and for trouble shooting. A design procedure is developed for a two-dimensional Markov chain to approximate the average run length (ARL) for the proposed chart. Results show that the MaxMin EWMA chart has good ARL properties for simultaneous changes in mean and standard deviation. A comparison is made with the combined use of X-bar and ln (S2) EWMA charts. An application is provided for 100 multi-stream processes in which the MaxMin EWMA chart has been successfully field tested and implemented.

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  • Topics: Quality Control, Design of Experiments
  • Keywords: Control charts,Quality control (QC),Nested experiments,Exponentially weighted moving average control charts (EWMA)
  • Author: Amin, Raid W.; Wolff, Hans; Besenfelder, Werner; Baxley, Robert, Jr.
  • Journal: Journal of Quality Technology