A new CUSUM-S2 control chart for monitoring the process variance

Purpose – The purpose of this paper is to introduce and investigate the performances of a newCUSUM-S 2 control chart designed to monitor the sample variance of samples from a normallydistributed population.Design/methodology/approach – The proposed chart monitors a statistic computed as alogarithmic transformation of the sample variance; the introduction of the sample variancelogarithmic transformation has a twofold effect: to quickly detect the occurrence of an “out-of-control”condition; to deal with a quasi-standard normal statistic.Findings – A design strategy trying to minimize the “out-of-control” average run length (ARL) of thechart is presented and the statistical performance of the CUSUM-S 2 chart has been assessed through acomparison with an EWMA-S 2 control chart proposed in the literature to monitor the processdispersion.Research limitations/implications – The paper only deals with uncorrelated normallydistributed data.Practical implications – The obtained results show how the CUSUM-S 2 chart is particularlysuitable when reduction in the process dispersion should be detected by means of subgroups havinglimited sample sizes.Originality/value – The paper shows the new CUSUM-S 2 control chart allows a decreasing of thevariability to be detected faster than the corresponding EWMA-S 2 control chart proposed earlier inthe literature.