Six Sigma has been hailed as one of the most powerful manufacturing performance tools, ever. The phrase was coined by Motorola in the 1980s to describe the company's approach to refining and improving all key production and business processes. The Six Sigma approach enabled Motorola to achieve very high levels of process capability. The results? A radical elimination of business and process costs resulting in massive increases in profitability. Since then, the Six Sigma approach has been sweeping the manufacturing world transforming the way businesses operate.

Six Sigma Defined

Generally there are two definitions of Six Sigma.

1. The literal definition. This is based on the statistical interpretation of process capability and referred to as 6σ. Sigma refers to the statistical term for standard deviation (sigma or σ), i.e. a measure of process distribution. A 6σ process operates to a process capability index (C _pk ) of at least 2 (i.e. the process spread, as defined by plus and minus 3 sigma about the mean setting, is at least half that of the design specification or tolerance). 6σ therefore means that the process setting or mean value is at least six standard deviations (or sigmas) from the nearest upper or lower design limit.

2. The generic term for the management approach to process improvement as developed by Motorola and others such as General Electric and Allied Signal. This is based on improving key business and product process performance to levels very near to perfection. If a production process operates to these levels, process defects can be counted in terms of parts per million. A process operating to Six Sigma will produce as few as 3.4 defects per million opportunities (DPMO). At the heart of Six Sigma is the use of multi-skilled team working and Design of Experiments (DoE).

Simplified Sigma Conversion table: