[1] Statisticians note: the area under the normal curve beyond Six Sigma is 2 parts-per-billion. In calculating error rates, Six Sigma presumes that over the long term processes will drift by as much as 1.5 sigma. Thus, you will find 3.4 PPM is the area beyond 4.5 sigma on the normal curve.

[2] This performance improvement model is analogous to the older TQM model known as PDSA, or Plan-Do-Study-Act

[i] Jeremy Main, Quality Wars: The Triumphs and Defeats of American Business, The Free Press, 1994, 173.

Thomas Pyzdek, Principal of Pyzdek Consulting, Inc. holds more than 50 copyrights including the Six Sigma Handbook, The Quality Engineering Handbook and The Handbook of Quality Management. His works are used by thousands of universities and organizations around the world to teach process excellence. Pyzdek has provided training and consulting to clients in all industries for over 23 years. He has worked in process excellence since 1967.