On the improvement of one-sided S control charts

Abstract

The most common charting procedure used for monitoring the variance of the distribution of a quality characteristic is the S control chart. As a Shewhart-type control chart, it is relatively insensitive in the quick detection of small and moderate shifts in process variance. The performance of the S chart can be improved by supplementing it with runs rules or by varying the sample size and the sampling interval. In this work, we introduce and study one-sided adaptive S control charts, supplemented or not with one powerful runs rule, for detecting increases or decreases in process variation. The properties of the proposed control schemes are obtained by using a Markov chain approach. Furthermore, a practical guidance for the choice of the most suitable control scheme is also provided.

Keywords:

• adjusted average time to signal
• adaptive control charts
• runs rules
• sample standard deviation
• process variation
• Shewhart control charts

Acknowledgements

The authors would like to thank the referees for their suggestions and comments which have improved the manuscript.