Specify methods of estimation for Nonnormal Capability Sixpack

Stat > Quality Tools > Capability Sixpack > Nonnormal > Estimate

Estimate parameters of the distribution

You can have Minitab estimate the parameters of the nonnormal distribution used for the capability analysis or you can choose to enter some or all of the parameters below.

  • Estimate parameters of distribution: Select to estimate the distribution parameters from the sample data. Minitab estimates any parameters that you do not specify below.
    Set shape
    Enter the shape or scale parameter, depending on the type of distribution you selected. The shape parameter affects the shape of the distribution, such as its skewness.
    Set threshold
    If you selected a 3-parameter distribution, enter the threshold parameter. The threshold parameter sets the minimum location of the data distribution.
    Note

    For more information on the shape, scale, or threshold of a distribution, go to Capability statistics for Nonnormal Capability Sixpack and click the parameter that you want to learn more about.

  • Use historical estimates: Select to specify historical estimates of the parameters. Enter constants or a column using the parameter order shown. The number of constants and rows in the column must equal the number of parameters in the distribution.

Methods of estimating within subgroup standard deviation for control charts

1 < Subgroup size ≤ 8
Select a method for estimating the within-subgroup standard deviation when the subgroup size is between 1 and 8.
  • Rbar: Rbar is the average of the subgroup ranges. This method is a common estimate of the standard deviation and works best with subgroup sizes from 2 to 8.
  • Sbar: Sbar is the average of the subgroup standard deviations. This method provides a more precise estimate of the standard deviation than Rbar, especially with subgroup sizes > 8.
  • Pooled standard deviation: The pooled standard deviation is the weighted average of subgroup variances, which gives larger subgroups more influence on the overall estimate. This method provides the most precise estimate of standard deviation when the process is in control.
Subgroup size > 8
Select a method for estimating the within-subgroup standard deviation when the subgroup size is greater than 8.
  • Sbar: Sbar is the average of the subgroup standard deviations. This method provides a more precise estimate of the standard deviation than Rbar, especially with subgroup sizes > 8.
  • Pooled standard deviation: The pooled standard deviation is the weighted average of subgroup variances, which gives larger subgroups more influence on the overall estimate. This method provides the most precise estimate of standard deviation when the process is in control.
Subgroup size = 1
Select a method for estimating the within-subgroup standard deviation when you have individual observations. When the subgroup size is 1, sample standard deviations or ranges within subgroups cannot be calculated. Instead, Minitab estimates the standard deviation using moving ranges.
  • Average moving range: The average moving range is the average value of the moving range of two or more consecutive points. This method is commonly used when the subgroup size is 1.
  • Median moving range: The median moving range is the median value of the moving range of two or more consecutive points. This method is best to use when data have extreme ranges that could influence the moving range.

Use unbiasing constants

Use unbiasing constants in the estimate of the within-subgroup standard deviation. This option applies to the Sbar, pooled standard deviation, and square root of MSSD methods.

Unbiasing constants reduce the bias that can occur when a parameter is estimated from a small number of observations. As the number of observations increases, unbiasing constants have less effect on the calculated results. Often, the choice to use unbiasing constants depends on company policy or industry standards.

Use moving range of length

Enter the number of observations used to calculate the moving range. The length must be ≤ 100. The default length is 2 because consecutive values have the greatest chance of being alike.

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