Editor’s note: This is part four of a four-part series about the history of quality. For a description of the earlier years in the quality movement, see part one, part two, and part three.

The Six Sigma methodology is used to identify and control variables that affect the output of a process. In his book Quality Planning and Analysis (McGraw-Hill, 4th ed. 2000), Frank M. Gyrna explains that Six Sigma “is a collection of managerial and statistical concepts and techniques that focus on reducing variation in processes and preventing deficiencies in product.” A process that is capable of operating at a Six Sigma level produces, on average, only 3.4 defects per million units.

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