## How is Sigma Estimated from a Control Chart?

Sigma – an important statistic for us to know. Sigma, or the standard deviation, is a measure of how much dispersion there is in a process. There are numerous ways to estimate sigma. One way, of course, is simply to calculate the value using the formula for the standard deviation:

where X i are the individual results, X is the average and n is the number of individual results.

This is a measure of the dispersion in the data. But is it a good one to use? Does it fairly represent the data? The answer is you don’t know because you haven’t...