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, c_{4}(*n*_{i}), then Sbar is calculated as follows:

When the subgroup size is constant, Sbar is:

## Notation

Term | Description |
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c_{4} (*n*_{i}) | value of the unbiasing constant c_{4} that corresponds to the value that is specified in parentheses. |

*S*_{i} | standard deviation of subgroup *i* |

*m* | number of subgroups |