Six Sigma (6σ) is a set of management techniques to manage quality and to improve processes and process output quality. Its key element is to describe, analyze, improve and monitor business processes using statistical methods.
As a rule, every quality characteristic leads to an undesired variation in process results. As a part of a so-called process capability inspection, deviations from the target state in relation to the tolerance level of the characteristics in question are identified. The standard deviation of the characteristic (the Greek letter σ, which is read as sigma) plays an important role. It measures the variance of the characteristic and how much the characteristics’ values deviate from one another. The further the standards deviate from the range of tolerance, the more likely it is that the tolerance levels will be exceeded. Also, the further the mean is from the middle of the tolerance range (the closer it is to one of the tolerance levels), the greater the regression. That is why it is important to measure the distance between the mean and the closest tolerance range using standard deviations. This distance, divided by σ is the process capability index cpk; Cpk = 1 if the mean is 3 σ from the closest tolerance level.
These techniques were named Six Sigma because Six Sigma requires that the closest tolerance level be at least six standard deviations (6σ, Six Sigma) from the mean. When this requirement is met, we can assume that a zero-error production has been met and that the tolerance levels are rarely exceeded.