If the column you enter for Standard deviations contains missing values, then a message asks whether you want to estimate the standard deviations. If you click Yes, then Minitab opens the Standard Deviation Estimator dialog box.

Use one of the methods below to enter values for each missing standard deviation. After you use the Standard Deviation Estimator, Minitab replaces the missing values in the worksheet with the estimated standard deviations.

To re-estimate the standard deviations, replace the estimated values with missing value symbols (*), then calculate the gap pools again.

- From Method, select Range (value/6.0).
- In Value, enter the expected range for the standard deviation of the element. Minitab assumes that the range is approximately equal to 6σ. Thus, dividing the range by 6 provides an estimate of σ.
- Click OK.

When you enter an upper confidence limit, the calculation is σ = (upper confidence limit – μ) / Zα.

When you enter a lower confidence limit, the calculation is σ = (μ – lower confidence limit) / Zα.

α-value | Zα equals |
---|---|

80 | 0.84162 |

90 | 1.28155 |

95 | 1.64485 |

99 | 2.32635 |

- From Method, select one of the confidence limit options.
- In Value, enter the confidence limit for the element.
- Click OK.

Although you can enter known standard deviations in the Standard Deviation Estimator, entering the values directly in the worksheet is easier.

- From Method, select Use value.
- In Value, enter the standard deviation for the element.
- Click OK.