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Special Cause Variation

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An unusual occurance in a process, causing it to behave erratically and the cause of which is usually under the control of the operator. Special cause variation is present in the system when a point shows as "out of control", or when unexpected patterns/trends appear 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. Also called Assignable cause variation.