Capability Sixpack (Normal Distribution) - Estimation of Standard Deviation
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Stat > Quality Tools > Capability Sixpack > Normal > Estimate

Allows you to specify the method for estimating the population standard deviation.

You can set preferences for the estimation of s using Tools > Options > Control Charts and Quality Tools > Estimating Sigma.

Whether Minitab displays an R chart or an S chart depends on the size of the subgroups. If more than 50% of the subgroups are the same size, then the most common subgroup size determines the chart. Otherwise, the largest subgroup size determines the chart. If the subgroup size is 8 or less, Minitab displays an R chart. If the subgroup size is 9 or more, Minitab displays an S chart. If the subgroup size is 1, Minitab displays a moving range chart.

Dialog box items

Methods of estimating within subgroup standard deviation

Capability Statistics: Choose an estimate of the within-subgroup standard deviation for the capability statistics.

(for subgroup size > 1)

Rbar: Use the average of the subgroup ranges.

Sbar: Use the average of the subgroup standard deviations.

Pooled standard deviation: Use a pooled standard deviation.

(for subgroup sizes = 1)

Average moving range: Use the average of the moving range.

Median moving range: Use the median of the moving range.

Square root of MSSD: Use the mean squared successive differences.

Control Charts: Choose an estimate of the within-subgroup standard deviation for the control charts

(for 1< subgroup size <= 8)

Rbar: Use the average of the subgroup ranges.

Pooled standard deviation: Use a pooled standard deviation.

(for subgroup size > 8)

Sbar: Use the average of the subgroup standard deviations.

Pooled standard deviation: Use a pooled standard deviation.

(for subgroup size = 1) Minitab generally estimates standard deviation using the sample standard deviations or ranges of each subgroup. But when the subgroup size is 1 you cannot calculate sample standard deviations or ranges. Minitab estimates standard deviation using moving ranges instead.

Average moving range: Use the average of the moving range.

Median moving range: Use the median of the moving range.

Use unbiasing constants to calculate standard deviations

Within subgroup: Check to estimate within-subgroup standard deviation using unbiasing constants.

Overall (capability statistics only): Check to estimate the overall standard deviation using unbiasing constants.  

Use moving range of length: Enter the number of observations used to calculate the moving range. By default, a span of 2 is used as consecutive values have the greatest chance of being alike.