Intervals & Tests
Although the distribution of choice for a number of control charts using plotted averages, and the most widely known distribution, it is not generally seen in practice as the distribution of observations. To fit a normal distribution, only the average and specified sigma (process, sample, or population) are needed. Note that the curve generated will be used for K-S values and process capability, but that process sigma is always used to define the distribution limits on the Individual-X chart.
Learn more about the Statistical Inference tools for understanding statistics in Six Sigma Demystified (2011, McGraw-Hill) by Paul Keller, in his online Intro. to Statistics short course (only $89) or his online Black Belt certification training course ($875).