Special Cause

(1) A cause of variation which is localized, or acts at one period of time, or in one set of circumstances. A cause not common to all outcomes of a process.

(2) Special cause variation is a source of variation that is intermittent, not predictable. Sometimes it is called "assignable cause" variation. On a control chart, a special cause is signaled by points beyond the control limits, runs, or nonrandom patterns within the control limits. A process that has special cause variation is said to be out-of-control, unstable, or unpredictable.