Quality & Metrology worked example

Ppk with distance from mean to nearest spec limit of 0.2 spec units: a worked example

Here is what the math looks like when conditions slip. We hold every other input steady and drop distance from mean to nearest spec limit to 0.2 spec units, then walk the calculation through step by step. Estimate process performance Ppk as the distance from the process mean to the nearest specification limit, divided by the one-sided long-term spread.

The inputs for this scenario

  • Distance from mean to nearest spec limit: 0.2 spec units (the input this scenario stresses; the baseline uses 0.4)
  • Required clearance buffer: 0.04 spec units (held at the documented default)
  • One-sided long-term spread (3 sigma): 0.3 spec units (held at the documented default)

Working through the calculation

  • The calculation starts from the formula this tool documents: Adjusted distance to limit = distance to nearest spec limit − required clearance buffer.
  • Ppk works out to 0.53 Ppk at these inputs, and this is the headline figure for the scenario.
  • Effective distance to limit works out to 0.16 spec units at these inputs.
  • One-sided spread (3σ) works out to 0.3 spec units at these inputs.

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 55.56% below the baseline at 0.53 Ppk.
  • The practical read: the gap between this scenario and the baseline is entirely attributable to distance from mean to nearest spec limit, so recovering it is worth quantifying in dollars before considering equipment or staffing changes. 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: 0.53 Ppk (headline result)
  • Effective distance to limit: 0.16 spec units
  • One-sided spread (3σ): 0.3 spec units

Run it with your numbers

  • To rerun this with your own numbers, open the live Ppk calculator, set distance from mean to nearest spec limit to your actual value, and adjust the remaining inputs to match your operation.

Last reviewed 2026-05-12.