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Sigma is the Greek letter assigned to represent standard deviation; that is, the amount of variation within a given process. The number of Sigma can be "fit" between the gap of process mean and the upper or lower specification limit, determine the defect level of a process. The higher the Sigma level, the lower the number of defect. Statistically, a process that achieving Six Sigma (6) level of quality equates to a mere 3.4 defects out of one million opportunities, or nearly defect free performance. The roots of Six Sigma as a measurement standard can be trace back to Carl Frederick Gauss ...