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 *MR _{i}* is the moving range for observation

Minitab uses to calculate *S _{mr}*, which is an unbiased estimate of

Term | Description |
---|---|

n | number of observations |

w | length of the moving range. The default is 2. |

d_{2}() | value of unbiasing constant d_{2} that corresponds to the value specified in parentheses. |

The median moving range, , of length *w* is given by the following formula:

where w is the number of observations used in the moving range and *MR _{i}* is the
moving range for observation

Minitab uses to calculate *S _{mr}*, which is an unbiased estimate of

Term | Description |
---|---|

n | number of observations |

w | length of the moving range. The default is 2. |

d_{4}() | value of unbiasing constant d_{4} that corresponds to the value specified in parentheses. |