Page 22 - Six Sigma Demystified

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c h a p t e r

Deployment Strategy

What Is Six Sigma?

Sigma (s) is the Greek letter used by statisticians to denote the standard de-
viation 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 require-
ments.
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 (s), 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 time in queue)
is 7.5 minutes with a standard deviation of the wait time calculated as 1 minute.
Six standard deviations, or 6s, from the average is 1.5 minutes (in the negative
direction) and 13.5 minutes (in the positive direction).
Separately, through the use of customer surveys, focus groups, or simple
feedback, customer requirements may have been established for the process. In
this case, the process is likely to have only an upper specification limit defined
by the customers; there is no minimum limit to desirable wait times.

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