Overview for Xbar-R Chart

Use Xbar-R Chart to monitor the mean and variation of a process when you have continuous data and subgroup sizes of 8 or less. Use this control chart to monitor process stability over time so that you can identify and correct instabilities in a process.

For example, a manufacturing engineer wants to determine whether the production process for a new plastic product is in control. Operators sample 5 units every hour for 20 hours and assess the strength of the plastic.

The points vary randomly around the center line and are within the control limits for both charts. No trends or patterns are present. The strength of the plastic product is stable across the 20 subgroups.

Where to find this control chart

To create an Xbar-R chart, choose Stat > Control Charts > Variables Charts for Subgroups > Xbar-R.

When to use an alternate control chart

  • For subgroups that have 9 or more observations, use Xbar-S Chart.
  • If there is a consistent source of variation within the subgroups, use I-MR-R/S Chart.
  • If you do not have subgroups, use I-MR Chart.
  • If your data are counts of defectives or defects, use an attribute control chart, such as P Chart or U Chart.