A New CUSUM-S2 Control Chart for Monitoring the Process Variance

摘要

In this paper the statistical performance of a CUSUM-S2 control chart designed to monitor the dispersion of a normally distributed process quality parameter is investigated. 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 eect: i) to quickly detect the occurrence of an out-of-control condition; ii) to deal with a quasi-standard normal statistic. A design strategy trying to minimize the out-of-control Average Run Lenght (ARL) of the chart is presented and the statistical performance of the CUSUM-S2 chart has been assessed through a comparison with an EWMA-S2 control chart proposed in literature to monitor the process dispersion. The obtained results show how the CUSUM-S2 chart is particularly suitable when a reduction in the process dispersion should be detected by means of subgroups having limited sample sizes.