Definition and context
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.
Where this term fits in the workflow
Cpk (Process Capability Index) is usually shown in the process control and risk.
Terms that help factories monitor variation, validate capability, and reduce risk before defects escape.
What is KaizenQ?
KaizenQ helps factory teams record inspection results, defect photos, and notes in one report shared with the office.
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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.