It is a metric because it encourages measurement of process performance.

Sigma () is the Greek letter representing a statistical unit of measurement that defines the standard deviation of a population . It measures the variability or spread of the data.

is also a measure of variability . It is a name given to indicate how much of the data falls within the customers ‘ requirements.

The terms “Sigma” is often used as a scale for levels of “goodness” or quality. Using this scale, “Six Sigma” equates to 3.4 defects per one million opportunities (DPMO).