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

Abstract

A previous article [9] described how a small number of subgroups can be used to set statistically sound control limits for X̄- 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 R̄ 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.