Cpk = 1.67: The PPAP Launch Target

Why does PPAP frequently require Cpk 1.67, and what happens to that margin once production ramps?

• Typical PPM: ≈ 0.273 PPM at the nearest limit; ≈ 0.545 PPM only for a perfectly centered, two-sided process
• Sigma equivalent: 5.01σ to the nearest specification limit

Common automotive practice uses 1.33 for established production and 1.67 for some initial or launch studies, but the controlling value is always the applicable customer-specific requirement, drawing, Control Plan, or internal standard. Do not treat these values as universal pass/fail rules.

1. Confirm 1.67 is the customer-specific PPAP requirement, not an internal default.
2. Design the study to detect the expected drop to steady-state Ppk.
3. Report Cpk 1.67 with study window, σ method, and MSA.
4. Plan the ongoing Ppk monitoring plan before submitting PPAP.

• Treating 1.67 as a permanent minimum when the CSR only requires it at launch.
• Not modeling the Cpk-to-Ppk shift when planning the study.
• Using 1.67 as an SPC control limit; it is a capability target, not a chart limit.

• The 1.67 reference exists because Cpk usually degrades to Ppk in production; the extra 0.34 is launch margin, not a stricter physical requirement.
• Reporting Cpk 1.67 at PPAP with no plan for the Cpk-to-Ppk decay is a common cause of first-quarter scorecard hits.
• Cpk 1.67 on a study of 100 parts within one hour is not proof of long-term Ppk 1.67 — the study window is the confounder.

• Do not apply 1.67 to non-critical characteristics unless the CSR requires it.

• Cpk 1.67 ≈ 5.01σ ≈ 0.29 PPM at the nearest tail.
• Typical Cpk-to-Ppk decay of 0.15–0.35 is common but not guaranteed.

Use the free Process Capability Calculator to compute Cp, Cpk, Pp, Ppk, sigma level, and predicted PPM from your measurement data — no sign-up required.

• Never present a Cpk 1.67 as if it is an ongoing production performance.

• ✓1.67 verified as the actual customer requirement, not an internal default
• ✓Study window and σ method disclosed
• ✓Plan for ongoing Ppk monitoring documented at PPAP time

• Process condition: statistical stability is required.
• Distribution assumption: approximately normal data.
• PPM convention: nearest specification-limit tail.
• Sigma-shift convention: no sigma shift applied.
• Cpk alone identifies distance to the nearest specification limit; it does not determine total two-sided defects unless centering or both tail distances are known.
• Predicted PPM is a model estimate, not a replacement for observed defect data.