Special Cause Variation

Variation caused by something going wrong with the process, producing a faulty output, the root cause fo which can be determined and rectified.

Special-cause variation is characterised by:

* New, unanticipated, emergent or previously neglected phenomena within the system;

* Variation inherently unpredictable, even probabilistically;

* Variation outside the historical experience base; and

* Evidence of some inherent change in the system or our knowledge of it.

Special-cause variation always arrives as a surprise. It is the signal within a system. Examples of special cause variation are:

* Poor adjustment of equipment

* Operator falls asleep

* Faulty controllers

* Machine malfunction

* Computer crashes

* Poor batch of raw material

* Power surges

* Broken part

* Operator absent