S chart

An S chart plots the process standard deviation 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 S 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 S 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 S 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.

Use the S chart when your subgroup size is 9 or more. You can use the R chart when your subgroup size is 8 or less.

Example:

A household products manufacturer wants to assess the stability of its filling process for a window cleaning product. Analysts sample ten window cleaner bottles every hour for 30 hours and weigh each of them.

S 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 fill weight of the window cleaner is stable across the 30 subgroups.