Sigma Level
Alternative process capability metric — measures the distance from the mean to the nearest specification limit in multiples of σ.
Also known as: sigma level, Z-score capability, DPMO
The sigma level (Z-value) expresses process capability as the number of standard deviations that fit between the mean and the nearest specification limit. It is the metric that gives Six Sigma its name.
- Z = 3σ — Cpk = 1.00 — approx. 66,800 ppm defective (short-term)
- Z = 4σ — Cpk = 1.33 — approx. 6,210 ppm
- Z = 6σ — Cpk = 2.00 — approx. 3.4 ppm (with the usual 1.5σ shift for long-term)
- ppm = parts per million defective
See also
Used in
In the Algorithm Lab
Sources
- Pyzdek, T. & Keller, P. — The Six Sigma Handbook, 5th Edition, McGraw-Hill
- Harry, M. & Schroeder, R. — Six Sigma: The Breakthrough Management Strategy, Currency/Doubleday
- Montgomery, D. C. — Introduction to Statistical Quality Control, 8th Edition