#### XBar & S Chart

A type of Subgroup Chart that shows mean and standard deviation. The XBar & S Chart plots subgroup mean on the upper chart (in exactly the same way as the XBar & R Chart), with out-of-tolerance points in a red color, and subgroup standard deviation on the lower chart. Two additional statistical parameters, CP and CPK, also appear on the chart.

This S Chart is considered a better measure of process variation than the R Chart, since it is less sensitive to special causes of variation that may produce only a single unusual value in the subgroup. When the subgroup size is greater than 25, this chart should always be used as the Subgroup Chart.

See also:

- Capability Indexes: Table of Calculations

- Control Limits for XBAR & S Charts

- Process Chart