Special Cause
A systematic influence beyond the process's normal common-cause variation — the trigger of a rule violation on a control chart.
Also known as: special cause, assignable cause
A special cause (assignable cause) is an identifiable event that produces a signal on the control chart: a point outside the limits or a pattern per Nelson / Western Electric rules.
- Typical examples: tool break, new operator crew, material change, faulty sensor, changed ambient conditions
- Defining feature: attributable to a concrete source (hence "assignable cause")
- Response: identify, document, and remove the cause — do not ignore, do not adjust the limits
See also
Used in
In the Algorithm Lab
Sources
- Shewhart, W. A. — Economic Control of Quality of Manufactured Product, 1931
- Deming, W. E. — Out of the Crisis, MIT Press, 1986
- Wheeler, D. J. — Understanding Variation: The Key to Managing Chaos, 2nd Edition, SPC Press