Modified S-charts for controlling process variability

To help in the detection of variance increases and decreases, three modified versions of traditional Shewhart S-charts are evaluated in terms of their average run length values. One scheme uses control limits based on equal tail chi-square distribution probabilities. The second uses control limits based on unequal tail probabilities. The third uses warning limits based on equal tail probabilities, but requires two successive points beyond the warning limit to give an out-of-control signal. They all result in better average run length values than the traditional S-chart. Also, if the only concern is the detection of variance increases, then both S-charts and warning limit charts without lower control limits are shown to have better average run length values than those of the traditional charts.