An Xbar-R chart plots the process mean (Xbar chart) and process range (R chart) over time for variables data in subgroups. This combination control chart is widely used to examine the stability of processes in many industries.
For example, you can use Xbar-R charts to monitor the process mean and variation for subgroups of part lengths, call times, or hospital patients' blood pressure over time.
The Xbar chart and the R chart are displayed together because you should interpret both charts to determine whether your process is stable. Examine the R chart first because the process variation must be in control to correctly interpret the Xbar chart. The control limits of the Xbar chart are calculated considering both process spread and center. If the R chart is out of control, then the control limits on the Xbar chart may be inaccurate and may falsely indicate an out-of-control condition or fail to detect one.
You can use the Xbar-R chart when your subgroup size is 8 or less. Use the Xbar-S chart when your subgroup size is 9 or more.
A plastics manufacturer wants to determine whether the production process for a new product is in control. Analysts 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.