The process standard deviation is also called sigma, or σ. If you enter an historical value for sigma, then Minitab uses the historical value. Otherwise, Minitab uses one of the following methods to estimate sigma from the data.
The average moving range, , of length w is given by the following formula:
where MRi is the moving range for observation i, calculated as follows:
Minitab uses to calculate Smr, which is an unbiased estimate of σ:
The median moving range, , of length w is given by the following formula: