Process capability index, the smaller of (USL minus mean) and (mean minus LSL), divided by 3 times the short-term standard deviation. It equals Cp only when the process is centered. A Cpk of 1.33 puts the nearest limit 4 sigma away, about 32 ppm out on that side; 1.67 puts it 5 sigma away. Many customers require 1.33 for ongoing production and 1.67 for safety or critical characteristics.
Customer wants a Cpk of 1.67 on that dimension and we're at 1.41. Either fix the fixture or ask for a wider tolerance.
Ask how sigma was estimated before you trust a Cpk: the same data can give 1.5 from within-subgroup sigma and 1.0 from overall sigma.
Also heard as
- C-P-K
- capability index
Do not mix it up with
- Ppk: Ppk uses the overall long-term standard deviation; Cpk uses the short-term within-subgroup estimate, so Ppk is usually lower.
Related terms
Cp
Process capability ratio: the specification width divided by six process standard deviations, ignoring centering.
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.
Sigma level
Process quality expressed as the number of standard deviations that fit between the mean and the nearest spec limit.
Specification limits
Acceptance boundaries for a product characteristic set by design or customer requirements, labeled USL and LSL.