R chart

An R chart plots the process range over time for variables data in subgroups. This control chart is widely used to examine the stability of processes in many industries.

For example, you can use R charts to examine process variation for subgroups of part lengths, call times, or hospital patients' blood pressure over time.

Examine the process variation using an R chart before interpreting the process average with an Xbar chart. The process variation must be in control to correctly interpret the Xbar chart because 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 R chart when your subgroup size is 8 or less. Use the S chart when your subgroup size is 9 or more.

Example:

A plastics manufacturer wants to determine whether the production process for a new product is in control. Analysts sample 5 products every hour for 20 hours and assess the strength of the plastic.

R chart

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