Six Sigma

Six Sigma is a structured methodology for reducing defects and variation in a process, built around the DMAIC cycle: Define the problem, Measure current performance, Analyze root causes, Improve the process, and Control it so gains hold. It originated in manufacturing at Motorola in the 1980s and was later popularized more broadly by General Electric.

The name refers to a statistical target: a process performing at "six sigma" produces about 3.4 defects per million opportunities, an extremely low error rate compared with typical process performance. Practitioners use statistical process control charts to monitor variation and tools such as fishbone diagrams and the "5 whys" for root cause analysis during the Analyze phase. Training is organized into belt levels (Green Belt, Black Belt, and so on) reflecting depth of statistical and project experience.

Six Sigma differs from Lean, which focuses on eliminating waste and speeding flow rather than statistically reducing variation; many organizations combine the two as "Lean Six Sigma." A common pitfall is applying full Six Sigma rigor to a process with too little data or too much natural variability for a defect-rate target to be meaningful, or treating belt certification as the goal rather than measurable defect reduction.