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. The engineer samples 5 units every hour for 20 hours and assess the strength of the plastic. The Xbar and R charts show that 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
PC: STATISTICS > Control Charts > Continuous > Xbar-R
Mac: Statistics > Continuous > Xbar-R
When to use an alternate control chart
For subgroups that have 9 or more observations, use Xbar-S Chart.