A statistical indicator showing how well a process stays within specification limits relative to its variation.
Cpk compares the distance from the process mean to the nearest specification limit with three times the within-process standard deviation. Interpret it only with the process stability, distribution and measurement system in view.
Calculate the index
Cpk = min((USL - mean)/(3σ), (mean - LSL)/(3σ)). State how σ was estimated. For a one-sided specification, use the relevant one-sided capability index.
Check the assumptions
Check process stability before assessing capability. Conventional defect-rate interpretations need appropriate distribution assumptions. The required capability threshold comes from the customer or control plan; 1.33 is not a universal acceptance rule.
Key takeaways
- Specification limits describe product requirements; control limits describe process behavior.
- A high Cpk from a short, favorable sample does not prove sustained capability.