How do CPK and GRR prove a test system can be trusted?
Use GRR first to measure how much of the tolerance the measurement system consumes: below 10% is acceptable, 10–30% is conditionally acceptable, above 30% is not acceptable, and the number of distinct categories should be at least 5. Then use Cpk or Ppk for process capability, where volume production usually requires Cpk ≥ 1.33. If GRR fails, a Cpk calculated from the same data cannot be used in a commitment to a customer.
Key points
- GRR below 10% acceptable, 10–30% conditional, above 30% unacceptable
- Number of distinct categories (NDC) should be 5 or higher
- Cpk ≥ 1.33 is the common volume production threshold
- A failed GRR invalidates the Cpk derived from the same data
- Both are preconditions for signing off a production test station
Products this applies to
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