A special cause is a source of variation that is not always present in a process and arises from a specific event or condition. It is also referred to as an assignable cause.
Examples: tool wear, incorrect setup, a new operator, a different raw material batch, equipment failure.
Difference from common cause
Common causes are inherent in the process and act continuously. Special causes come from outside and produce a signal on the control chart.
Why does it matter?
The distinction determines the intervention decision. When a special cause is detected, its cause is found and eliminated. Intervening one by one in common cause variation, on the other hand, makes the process worse.
Every intervention made without making this distinction actually increases variation. The most common mistake on the shop floor is mistaking normal fluctuation for a special cause and adjusting.
Capability indices are not calculated until special causes have been eliminated; the process must first be brought into statistical control.
Special cause detection rules and signal interpretation are addressed hands-on in the SPC training. For finding the cause, see also the CQI-20 effective problem solving programme.