Process capability ratio, (USL minus LSL) divided by 6 times the short-term standard deviation. A Cp of 1.0 means the natural 6 sigma spread exactly fills the tolerance, about 2,700 ppm out of spec even when perfectly centered; 1.33 means the tolerance holds 8 sigma. Because Cp ignores where the mean sits, it shows potential capability: what the process could achieve if the mean were put on target.
Cp is 2.1 but Cpk is 0.9, so the spread is fine. It's running off center, retarget it and you're done.
Also heard as
- process capability ratio
- process potential index
Do not mix it up with
- Cpk: Cpk penalizes an off-center mean; Cp assumes the process is centered.
Related terms
Cpk
Process capability index that accounts for centering: distance from the mean to the nearest spec limit in units of 3 sigma.
Ppk
Process performance index: a Cpk-style figure computed with the overall standard deviation, capturing long-term variation.
Process capability
How well a stable process's natural spread fits inside the specification limits, usually expressed as an index.
Specification limits
Acceptance boundaries for a product characteristic set by design or customer requirements, labeled USL and LSL.