PPM (Parts Per Million)
Definition
A measure of defect rate expressed as defective units per million produced. Calculated from the probability of measurements falling outside specification limits under normal distribution assumptions.
Practical Note
Cpk 1.0 ≈ 2700 PPM, Cpk 1.33 ≈ 63 PPM, Cpk 1.67 ≈ 0.6 PPM, Cpk 2.0 ≈ 0.002 PPM. PPM is preferred over percentage for high-quality processes where defects are rare.