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On Proper Choice of Variability Control Chart for Normal and Non-normal Processes


Saddam Akber Abbasi, Department of Statistics, The University of Auckland, Auckland, New Zealand.



Control charts are an important statistical process control tool used to monitor changes in process location and variability. This study addresses issues regarding the proper choice of control chart for efficient monitoring of process variability. The choice of the best estimator to be used for variability charts has not been made clear in literature. We have analyzed the performance of eight control chart structures, based on different estimates of process standard deviation. The performance of control charts is investigated under the existence and violation of ideal assumptions of normality. Control chart constants and factors required for computing probability limits, considering normal and different non-normal parent distributions, are provided for all variability charts. This study aims at providing guidance to quality practitioners in choosing the appropriate variability control chart for normal and non-normal processes. Copyright © 2011 John Wiley & Sons, Ltd.