Methods and formulas for estimating sigma for Xbar-S Chart

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.

Without unbiasing constant

If you do not use an unbiasing constant, then the Sbar is the mean of the subgroup standard deviations:

With unbiasing constant

If you use the unbiasing constant, c4(ni), then Sbar is calculated as follows:

When the subgroup size is constant, Sbar is:


c4 (ni)value of the unbiasing constant c4 that corresponds to the value that is specified in parentheses.
Sistandard deviation of subgroup i
mnumber of subgroups
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