6 sigma methodology was introduced in 1986 by Motorola company, to resolve quality related problems. 6 sigma gained popularity among manufacturers in 1995 after Jack Welch made it principal business strategy at General Electric.
The name of methodology arises from greek letter σ (sigma). In statistics it means a standard deviation from the mean. Achieving sixth sigma level is equivalent to perfect quality.
| Sigma level | DPMO* | Defects | Yield |
| 1σ | 691 462 | 69% | 31% |
| 2σ | 308 538 | 31% | 69% |
| 3σ | 66 807 | 6,7% | 93,3% |
| 4σ | 6 210 | 0,62% | 99,38% |
| 5σ | 233 | 0,023% | 99,977% |
| 6σ | 3,4 | 0,00034% | 99,99966% |
* DPMO – Defects per million opportunities