The Xbar and R Chart is located in the upper left corner of the Capability Sixpack - Normal.

Use the -R chart, Xbar-S chart, or the I-MR chart to assess whether the process is in control. The process should be in control before you assess capability. If the process is not in control then the capability estimates will be inaccurate. Which chart is displayed depends on the subgroup size:

·    When your subgroup size is greater than one and less than nine, an -R chart is displayed. When your subgroup size is greater than or equal to nine, Xbar – S chart is displayed.

-   The chart plots the subgroup means. Use the chart to determine whether the average subgroup measurements are in control.

-   The R chart plots the subgroup ranges. Use the R chart to determine whether the subgroup-to-subgroup variation is in control.

-   The S chart plots the subgroup ranges. Use the S chart to determine whether the subgroup-to-subgroup variation is in control.

·    When your subgroup size is one, an I-MR chart is displayed.

-   The individuals (I) chart plots the individual observations. Use the I chart to assess whether the individual measurements are in control.

-   The moving range (MR) chart plots moving ranges for consecutive observations. Use the MR chart to assess whether the variation from observation to observation is in control.

Examine the -R chart or I-MR chart for points located beyond the red control limits. Points located beyond the control limits signal that the process is out of control. Capability analyses should only be conducted when the process is in control.

For the piston data, the subgroup size is 5, so an -R chart is displayed. No points are located beyond the control limits, therefore the process is in control, and further capability analyses can be conducted.