Variation resulting from an assignable cause. Special causes should be addressed by finding the special cause and taking action. Special Causes can be beneficial, in which case identify the cause and seek to incorporate it in the standard practice. The two most common mistakes that are made in reducing, understanding, or treating variation is treating a common cause as special and a special cause as common. It is variation that can be assigned to an identifiable source whereas Common Cause Variation is difficult to link to any single source.(JJH)
(see also Common Cause Variation)
Special Cause Variation – Unlike common cause variability, special cause variation is caused by known factors that result in a non-random distribution of output. Also referred to as “exceptional” or “assignable” variation. Example: Few X’s with big impact.
Special cause variation is a shift in output caused by a specific factor such as environmental conditions or process input parameters. It can be accounted for directly and potentially removed and is a measure of process control.
(iSixSigma.com dictionary) (see also Common Cause Variation)