Common Cause Variation

Common cause variation is variation that is inherent to a process, caused by unknown factors, resulting in a random but well-defined distribution of process measurements. It describes the natural noise in a process and is a direct result of how the process is designed, constructed, and managed.

A process that is subject only to common cause variation can be regarded as stable and predictable, in the sense that the process average and process variation can computed and used to predict the future behavior of the process. However, even though the process variation can be regarded as stable, it may still be too large to be acceptable. For example, a stable process, subject only to common cause variation, may be consistently producing output that is too large or too small. If the magnitude of the process variation allows the output to meet requirements, the process is said to be capable. If the magnitude of the process variation is so large that requirements are consistently not met, the process is said to be not capable. A reduction in common cause variation requires a change to the process itself and is generally the responsibility of management.

A control chart displaying measurements from a process that is subject only to common cause variation displays a random pattern, almost always with all points within the control limits, and with no runs, shifts, or trends. Points that occur outside the control limits, or which display shifts or trends, are regarded as due to special cause variation.

For patients seeking care in a walk-in clinic, the amount of time it takes for a patient to be seen by a physician is variable. It could depend on a number of unpredictable factors, including the number of other individuals seeking care, the nature of care required, the staffing, and so on. Taken as a whole, these different sources of variation constitute the common cause variation in this process. If something extraordinary happens, say all the physicians run out of the clinic to take care of the victims of a traffic accident, then the wait times become extremely long, and this is called special cause variation.