Sigma (σ) is the Greek letter used by statisticians to denote the standard deviation for a set of data. The standard deviation provides an estimate of the variation in a set of measured data. A stated sigma level, such as Six Sigma, is used to describe how well the process variation meets the customer’s requirements.

Figure 1.1 illustrates the Six Sigma level of performance for a stable process. The process data are represented by the bell-shaped distribution shown. Using the calculated value of the standard deviation (σ), the distance from the process centerline to any value can be expressed in sigma units. For example, consider the teller station at a bank whose average customer wait time (or ...

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