From IPAPI

Six Sigma is a project-focused improvement practice based on the five steps of Define, Measure, Analyze, Improve and Control (DMAIC).

IPAPI Definition

Six Sigma – Six Sigma is a project-focused improvement practice based on the fixing of effects loosely derived from the originating statistical manufacturing method.

Overview

Six Sigma originated as a statistical manufacturing control method. The observation was made that by controlling machinery at a target tolerance range significantly narrower (or tighter) than acceptable tolerances, the natural variation from influencing factors in machining functions that could produce “out of specification” parts could be controlled to “guarantee not more than X bad parts per million” produced.

Later, Six Sigma became associated with solving problems and managing projects through a structured methodology in an attempt to apply the “rudimentary quality concept” of Six Sigma for machining to other work and later processes in the organization. The Six Sigma practice relating to processes is the prominent use of Six Sigma today and is centered on doing things better – or as we like to say – doing things right.

Once outside the domain of manufacturing, Six Sigma has proven to have far less value than originally perceived, with return on investment now becoming quite difficult to justify in the majority of cases. This lack of return from Six Sigma is what has spurred the generation of Lean.