In a normally distributed process, 99.73% of measurements will fall within +/- 3.0 sigma, and 99.99966% will fall within +/- 4.5 sigma. A process with a normal distribution and normal variation of the mean, would need to have a specification limits of +/- 6.0 sigma in order to produce less than 3.4 defects per million opportunities – this is Six-Sigma.
The roots of Six Sigma as a measurement standard can be traced back to Carl Frederick Gauss, who introduced the concept of the normal curve. Six-Sigma, as a Total Quality Management (TQM) implementation is a concept that originated in Motorola. Bill Smith, of Motorola, is considered as the “Father of Six Sigma” movement.
In 1986 Bill Smith introduced the concept of Six Sigma to standardize the way defects are counted. In Smith’s words, “We recognized that the issue was not building a single part with one characteristic, it was building a product with thousands of parts. If you reduce the total defects, you increase customer satisfaction and reduce costs.”
When operations reach Six-Sigma quality, defects becomes so rare that when they do occur, they get into limelight and thus receive full attention necessary to determine and correct the root-causes.
Six-Sigma implementation is a process generally defined by its acronym “DMAIC”, viz.: