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

Abstract

Purpose

The purpose of this paper is to introduce and investigate the performances of a new CUSUM‐S² 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‐S² chart has been assessed through a comparison with an EWMA‐S² 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‐S² 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‐S² control chart allows a decreasing of the variability to be detected faster than the corresponding EWMA‐S² control chart proposed earlier in the literature.

Citation

Castagliola, P., Celano, G. and Fichera, S. (2009), "A new CUSUM‐S² control chart for monitoring the process variance", Journal of Quality in Maintenance Engineering, Vol. 15 No. 4, pp. 344-357. https://doi.org/10.1108/13552510910997724