Six-Sigma:

Six sigma means literally six standard deviations. The phrase refers to the limits drawn on statistical process control charts used to plot statistics from samples taken regularly from a production process. Consider the process mean. A process is deemed to be “in control” at any given point in time if the mean of the sample at that time is within six standard deviations of the overall process mean to that point. In this case, “standard deviation” means the standard deviation of the sample mean. Six sigmas (= six standard deviations) is a very broad range, and the use of six-sigmas, rather than 3-sigmas, was popularized by Motorola. It poses substantial demands on the manufacturing process to limit variability of output so that a six-sigma-wide band lies within the limits of an acceptable process.

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