Options - Box-Cox
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Charts  All except the Attributes charts and the Rare Event Charts

You can use the Box-Cox transformation when your data are very skewed or when the within-subgroup variation is unstable to make the data "more normal." The transformation takes the original data to the power l, unless l = 0, in which case the natural log is taken. (l is lambda.)

To use this option, the data must be positive. When you include or exclude rows using control chart Options > Estimate, Minitab only uses the non-omitted data to find lambda.

The Options subdialog box lists the common transformations natural log (l = 0) and square root (l = 0.5). You can also choose any value between - 5 and 5 for l. In most cases, you should not choose a l outside the range of -2 and 2. You may want to first run the command Stat > Control Charts > Box-Cox Transformation to help you find the optimal transformation value.

When you use this transformation, Minitab does not accept any values you enter in the Parameters tab for historical means or historical standard deviations.  

Minitab uses rounded values for Box-Cox transformations when possible. If do not want to use rounded values for l, see Tools > Options > Control Charts and Quality Tools > Other.  

Dialog box items

Use a Box-Cox transformation (W = Y**Lambda): Choose to use when your data are very skewed or when the within-subgroup variation is unstable.

Lambda = 0 (ln): Choose to use the natural log of the data.

Lambda=0.5 (square root): Choose to use the square root of the data.

Optimal lambda: Choose to have Minitab search for an optimal value.  

Optimal lambda for each stage (in a chart with stages): Choose to have Minitab search for optimal values for each stage.

Other (enter a value between -5 and 5): Choose to transform the data using another lambda value, then enter the lambda value.