What does an out-of-control process indicate?
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.
