Stable process

only common-cause variation

A process is stable (in statistical control) when its control chart shows only common-cause variation: no points outside the limits and no patterns. It is predictable, so capability indices describe its future performance.

Stable does not mean good. A stable process can still produce scrap if it is off target or too wide.

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
• Special cause: assignable cause of variation
• Control limits: voice of the process