A variable sample size S2-EWMA control chart for monitoring the process variance

Monitoring the stability of measures dispersion from a process quality parameter is animportant aspect of Statistical Process Control which should be carefully planned bypractitioners. To perform this task, this paper proposes an adaptive EWMA chart asa practical and efficient tool. The developed EWMA chart is the Variable Sample Size(VSS) version of a static S2-EWMA control chart previously developed by one of theauthors to monitor the sample variance. The way to compute the design parameters ofthis VSS S2-EWMA control chart is discussed and an optimal design strategy basedon the Average Time to Signal (ATS) after a shift in process dispersion is presented.The statistical performance of the VSS S2-EWMA has been evaluated by means of acomparison with two other EWMA charts: the static S2-EWMA and the adaptive (VSI)S2-EWMA allowing to vary the sampling intervals. The obtained results show how thepossibility of varying the sample size significantly improves the statistical performanceover the static S2-EWMA; furthermore, some interesting findings suggest to implementthe VSS S2-EWMA with respect to the VSI S2-EWMA when some particular processoperating conditions occur