Six Sigma is a data-driven improvement methodology that reduces variation and defects in processes. The name refers to the goal of a maximum of 3.4 defects per million opportunities — six standard deviations between the process mean and the nearest tolerance limit.
Where Lean chases waste and flow, Six Sigma chases variation: two deliveries can both be “within tolerance”, but unstable variation makes the process unpredictable and creates hidden costs. The methods are often combined as Lean Six Sigma.
Six Sigma is classically organised with belt grades (Yellow, Green, Black, Master Black Belt) that indicate method competence and project responsibility.