Cpk relates the distance between the process mean and the nearest spec limit to process variation. Cpk ≥ 1.33 is a common minimum target for critical characteristics; ≥ 1.67 for safety-critical attributes. Ppk uses overall variation and is used for initial capability, while Cpk uses within-subgroup variation for ongoing control. Capability studies need enough data, often at least 30 samples, and data that is roughly normal or analyzed with suitable methods. In process validation, capability targets are tied to the risk of the characteristic and are often set in the PQ protocol.

Why it matters

PPAP, supplier scorecards, and process validation routinely use Cpk as a lever; 'Cpk theater' (manipulated samples) is a recurring audit concern.

Common pitfalls

•Using Cpk as the sole metric for process control without considering historical data or other statistical process control tools can be misleading.
•Manipulating samples to achieve a favorable Cpk without actual process improvement constitutes "Cpk theater" and can lead to non-conformances during audits.
•Assuming a good Cpk guarantees product quality without addressing potential sources of variation or measurement system analysis is a common error.
•Failing to establish appropriate Cpk targets based on risk and product criticality can result in inadequate process control for critical attributes.
•Not periodically re-evaluating Cpk after process changes or over time can lead to a loss of process control without detection.

Sources

AIAG SPC Manual · AIAG
GHTF/IMDRF Process Validation Guidance · IMDRF/GHTF
FDA - Device Manufacturing · FDA

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