Six Sigma

refers to a quality management methodology. The name is derived from the claim that the tolerance limits of a normally distributed (production) process are at least 6 standard deviations (6?) away from the optimum. This results in a maximum defect rate of 3.4 defective parts per 1 million parts. 6? also stands for a wealth of statistically based process models for quality assurance.

Source: logipedia / Fraunhofer IML