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Special causes of variation

When something happens to cause a variation in the results or output of a process, these may be caused by factors that are constantly present in the process or they may be caused by unusual factors, often from outside the process. These latter causes are called special causes of variation.

Special causes of variation can be eliminated by reacting to individual variations (unlike common causes of variation).

Control Charts can be used to differentiate between common and special causes of variation.


Special causes of variation are sometimes called Assignable Causes as they may reliably be assigned to a single cause. They are also called Black Noise in allusion to the bad effects they can cause.

See also:

Control Chart, Common causes of variation, Variation

Toolbook chapter: Variation

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