Random variation in operating conditions that is outside the control of operators or equipment controls. Examples of sources of common cause variation are air temperature, weather conditions, random variation in the process, etc.
When only common-cause variation is present in a system, all points on a statistical process control chart are be within the 3-sigma control limits, and no unexpected patterns are detected on the charts.
In contrast, special cause variation is present in the system when a point goes out of control, or when unexpected patterns exist on a statistical process control chart. When this happens, the source of the variation should be determined to find a root cause which can be addressed and eliminated.
Statistical Process Control
Special Cause Variation