Enhancements for the S2 Control Chart

The S2 control chart is often used in the monitoring of shifts in the process variance. The S2 chart is quick in detecting big shifts but is less sensitive to small shifts. This paper aims at increasing the sensitivity of the S2 chart in the detection of small shifts while maintaining its sensitivity towards big shifts. Numerous runs rules schemes will be considered and incorporated into the conventional S2 chart. The suggested approch involves a simple transformation of the S2 statistics which does not complicate the application of the chart by industrial practitioners and engineers.