(X)BAR-Q AND RQ CHARTS - ROBUST-CONTROL CHARTS

When a process is first subjected to statistical quality control procedures, a typical approach is to collect 20-40 subgroups of observations and then to construct control charts such as XBAR and R charts with limits determined by the data. These control charts are then used to detect problems in control, such as outliers or excess variability in subgroup means, that may be due to a special cause, In this paper, I present a new method of determining the control limits for XBAR and R charts that results in easier detection of outliers and greater sensitivity to other forms of out-of-control behavior when outliers are present. The control limits for these X(Q)BAR and R(Q) charts are determined from the average subgroup interquartile range. They are nearly as easy to construct as ordinary XBAR and R charts.