XR and XS Subcommands (SPCHART command)

XR produces an X-bar chart and an R chart. XS produces an X-bar chart and an s chart. X-bar, R, and s charts are control charts for continuous variables, such as size, weight, length, and temperature.

An X-bar chart plots the mean of each subgroup. The center line indicates the mean of subgroup means. The control limits are calculated from subgroup means, numbers, standard deviations, and the user-specified SIGMAS value. The following figure shows an X-bar chart.

An R chart plots range values (maximum-minimum) of successive subgroups. The center line indicates the mean of subgroup ranges. The control limits are calculated from subgroup ranges, numbers, and the user-specified SIGMAS value. The R chart tests whether the process variability is in control. When the subgroup size is relatively small (4, 5, or 6), the range method yields almost as good an estimator of the variance as does the subgroup variance. The following figure shows an R chart.

An s chart plots subgroup standard deviations. The center line indicates the mean of subgroup standard deviations. The control limits are calculated from subgroup standard deviations, numbers, and the user-specified SIGMAS value. The s chart tests whether the process variability is in control, especially when the subgroup size is moderate to large. The following figure shows an s chart.