Special cause

assignable cause of variation

A source of variation that is not part of the normal process, for example a broken tool, a new material batch or a wrong setting. It shows up as a signal on the control chart and should be found and removed.

Calculate it

Process Capability & Control Charts: Cp, Cpk, Pp, Ppk, PPM and confidence intervals with X-bar/R, X-bar/S or I-MR charts and all 8 Nelson rules.

Related terms

• Common cause: natural, random variation
• Nelson rules: tests for special causes on control charts