A NewS2-EWMA Control Chart for Monitoring the Process Variance

This article demonstrates how a three-parameter logarithmic transformation combined with an exponentially weighted moving average (EWMA) approach can be used to monitor the sample variance of a process. The computation of the parameters of the logarithmic transformation and the control limits are explained. An easy-to-use table is provided and an illustrative example is given. The performance of the logarithmic transformation is evaluated under the assumption of normality. An optimal design strategy based on the ARL is presented and comparisons with other procedures are performed. Copyright © 2005 John Wiley & Sons, Ltd.