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 million or 3.4 allowing for statistical variation (whether it is so low in practice when claimed is the subject of some skepticism.) Six Sigma has fallen out of favour and been replaced by continuous improvement or Kaizen 改善 a principle derived from Japanese industrial practices which seek zero defects.