6-Sigma (6-σ)

Internal quality control system and standard originally developed by Motorola Corporation and subsequently adopted by General Electric and numerous other companies. 6-σ is a statistically driven method for identifying and correcting sources of quality problems within a manufacturing or service delivery process. Occasionally references to complying or providing data for 6-σ methodologies are found in supply and service contracts.

Sigma (σ) is the term or symbol used for a standard deviation (i.e., the margin of error in measuring a quantity). In a standard distribution, 68 percent of results will lie within 1 standard deviation of the mean, while 99.99999998 percent of results lie within 6-σ of the mean; thus in a 6 sigma process the defect rate will, in principle, be 2 per billion (whether it is so low in practice is the subject of some skepticism.)