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 new CUSUM‐ S 2 control chart designed to monitor the sample variance of samples from a normally distributed population. Design/methodology/approach The proposed chart monitors a statistic computed as a logarithmic transformation of the sample variance; the introduction of the sample variance logarithmic 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 the chart is presented and the statistical performance of the CUSUM‐ S 2 chart has been assessed through a comparison with an EWMA‐ S 2 control chart proposed in the literature to monitor the process dispersion. Research limitations/implications The paper only deals with uncorrelated normally distributed data. Practical implications The obtained results show how the CUSUM‐ S 2 chart is particularly suitable when reduction in the process dispersion should be detected by means of subgroups having limited sample sizes. Originality/value The paper shows the new CUSUM‐ S 2 control chart allows a decreasing of the variability to be detected faster than the corresponding EWMA‐ S 2 control chart proposed earlier in the literature.