Sigma (σ) is the standard deviation of the process. If you enter an historical value for σ, then Minitab uses the historical value. Otherwise, Minitab uses one of the following methods to estimate σ from the data.
Minitab uses the range of each subgroup, , to calculate , which is an unbiased estimator of σ:
When the subgroup size is constant, the formula simplifies to the following:
where (Rbar) is the mean of the subgroup ranges, calculated as follows:
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: