What does an out-of-control process indicate?

Many believe that an out of control process produces defective parts. That’s not always true.

Control charts are one of the most popular SPC tools used by manufacturers. They are used to determine whether a process is in or out of control.

When points on a control chart move outside the upper or lower control limit, the process is said to be “out of control.” As long as the points are within control limits, the process is “in control.” But, what does an out of control process indicate? Many believe that an out of control process produces defective parts. That’s not always true.

The control chart in the figure below illustrates points outside of both the lower control limit (LCL) and the upper control limit (UCL), indicating that the process is out of control. However, the points remain within the specification limits. Because the measurements are well within specifications, the parts are not defective.

Actually, an out of control process indicates the presence of non-random variation. Non-random variation is caused by definite, specific causes that are called assignable causes. These assignable causes make the process go out of control or become statistically unstable.

The presence of an out of control condition should prompt further investigation. It is important to find the assignable causes and take action to remove them. Once assignable causes are removed and only random variation due to common causes remains, the process will become stable and return to an in control condition.

Steven Wachs, Principal Statistician
Integral Concepts, Inc.

Integral Concepts provides consulting services and training in the application of quantitative methods to understand, predict, and optimize product designs, manufacturing operations, and product reliability. www.integral-concepts.com