StATS: What is a common cause of variation?

A common cause of variation is a variation from the mean that is caused by the system as a whole. This variation is not due to an assignable cause, but rather represents variation inherent in the process you are studying.

When a work process has only common causes of variation and no special causes, that process is "in control." This means that it is stable, consistent, and predictable. It might be predictably good or predictably bad, or it might be a very regular mix of good and bad results.

What do you do with a common cause of variation?

Just because a process is stable, or in statistical control, does not mean that its results are satisfactory. A process may be very consistent, day in and day out making items that are nowhere near specification limits. Or, as the Japanese have done so successfully, variation can be systematically reduced, even in stable processes, enabling a gradual tightening of specification limits, and an overall increase in product quality at lower cost.

Improving a stable process is somewhat more difficult than improving an unstable process because, by definition, a stable process has no special causes of variation that jump out at you, asking to be investigated. Instead, you are faced with the task of looking at all data about the process, not simply what made one point different from the others.

Common causes of variation often lie hidden within the system, and are sometimes assumed to be unavoidable. Yet it is very possible, and often very rewarding, to improve processes and reduce common cause variation. Experience had shown that, amongst the people in and around the process, there are enough ideas for improvements to make a significant impact, even on a sound process. (Source:

This page was written by Steve Simon while working at Children's Mercy Hospital. Although I do not hold the copyright for this material, I am reproducing it here as a service, as it is no longer available on the Children's Mercy Hospital website. Need more information? I have a page with general help resources. You can also browse for pages similar to this one at Category: Control charts, Category: Definitions.