The process standard deviation is also called sigma, or σ. If you enter a historical value for sigma, then Minitab uses the historical value. If you do not enter a value, Minitab uses one of the following methods to estimate sigma from the data.
If you do not use an unbiasing constant, then the Sbar is the mean of the subgroup standard deviations:
If you use the unbiasing constant, c4(ni), then Sbar is calculated as follows:
When the subgroup size is constant, Sbar is:
The pooled standard deviation (Sp) is given by the following formula:
When the subgroup size is constant, Sp can also be calculated as follows:
By default, Minitab applies the unbiasing constant, c4(), when you use the pooled standard deviation to estimate σ: