Distance from the mean is calculated in standard
deviations, and the symbol used for standard deviation is
the greek letter sigma
(
).
If you know every single value in the population, you can calculate the standard deviation by following a simple formula. However, even if you only know some of the values, you can still estimate the standard deviation of the entire population. There are several ways to estimate the standard deviation, and one of them is used in the calculations for the limits on a control chart. The reason control charts use the standard deviation will be shown on the next page.