Determining sigma levels of processes (one sigma, six sigma, etc.) allows process performance to be compared throughout an entire organization, because it is independent of the process. It is merely a determination of opportunities and defects, however the terms are appropriately defined for that specific process.

Sigma is a statistical term that measures how much a process varies from perfection, based on the number of defects per million units.

One Sigma = 690,000 per million units

Two Sigma = 308,000 per million units

Three Sigma = 66,800 per million units

Four Sigma = 6,210 per million units

Five Sigma = 230 per million units

Six Sigma = 3.4 per million units

In formulae for control limits and process capabilities, sigma is the symbol for Standard Deviation, calculated from the squares of the deviations of measured samples from the mean value (or sometimes by other methods using ‘magic’ numbers). For a normally distributed output, 99.7% would be expected to fall between +/-(3 x sigma) levels.

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