A defect rate metric expressing how many units out of one million are non-conforming.
PPM expresses a count per million units. State whether the count is nonconforming units or individual nonconformities; these measure different things.
Calculate nonconforming-unit PPM
If 12 of 40,000 inspected units are nonconforming, PPM = 12 / 40,000 × 1,000,000 = 300. Count each nonconforming unit once, even if it has several defects. This is an illustrative calculation.
Keep the denominator consistent
Compare periods only when inspection coverage, product scope and counting rules match. Sample results describe the inspected sample; do not present them as a full-lot count. Investigate changes in product mix or inspection before attributing a decrease to corrective action.
Key takeaways
- Record the numerator, denominator and inspection scope with every PPM result.