Mean and Variance Control Chart Limits Based on a Small Number of Subgroups

Summary: A previous article [9] described how a small number of subgroups can be used to set statistically sound control limits for X-bar- and R-charts. However, if the power of the charts is of paramount importance, and computational considerations are secondary, then it may be worthwhile to replace R-bar by more precise statistical measures of the dispersion of the process. This article considers two such measures, the average subgroup variance and the sample variance, and shows for each how to obtain statistically sound control limits based on a small number of subgroups.