Quality & Metrology worked example

Ppk with distance from mean to nearest spec limit of 1 spec units: a worked example

What does the result look like when distance from mean to nearest spec limit reaches 1 spec units? The full calculation is worked below with real intermediate numbers. Use it when reporting overall, long-term performance of an off-center feature for a PPAP or capability submission.

The inputs for this scenario

  • Distance from mean to nearest spec limit: 1 spec units (raised for this scenario; the documented default is 0.4)
  • Required clearance buffer: 0.04 spec units (unchanged)
  • One-sided long-term spread (3 sigma): 0.3 spec units (unchanged)

Working through the calculation

  • Applying the documented formula (Adjusted distance to limit = distance to nearest spec limit − required clearance buffer) to the inputs above produces each figure below.
  • At this operating point the engine returns 3.2 Ppk for ppk, the number this scenario is built around.
  • At this operating point the engine returns 0.96 spec units for effective distance to limit.
  • At this operating point the engine returns 0.3 spec units for one-sided spread (3σ).

How this compares with the baseline

  • Against the tool's baseline example, where distance from mean to nearest spec limit sits at 0.4 spec units and the headline result is 1.2 Ppk, this scenario comes in 167% above the baseline at 3.2 Ppk.
  • A figure at this level is achievable when distance from mean to nearest spec limit is genuinely sustained, not just peaked for a shift. Ppk only reflects the nearest limit, so a badly skewed or bimodal distribution can post an acceptable Ppk while still producing defects at the far tail; verify the assumption of normality.

Results at a glance

  • Ppk: 3.2 Ppk (headline result)
  • Effective distance to limit: 0.96 spec units
  • One-sided spread (3σ): 0.3 spec units

Run it with your numbers

  • Every input above is editable in the live Ppk calculator, which recalculates instantly and can be shared with the inputs intact.

Last reviewed 2026-05-12.