Box-Cox Transformationoverview how to example data see also
Box-Cox TransformationStat > Control Charts > Box-Cox Transformation
Performs a Box-Cox procedure for process data used in control charts. To use Box-Cox, all data must be greater than 0.
When you ask Minitab to estimate lambda, you get graphical output. See Box-Cox Transformation - Graphical Output for an explanation of the output.
The Box-Cox transformation can be useful for correcting both nonnormality in process data and subgroup process variation that is related to the subgroup mean. Under most conditions, it is not necessary to correct for nonnormality unless the data are highly skewed. Wheeler [34] and Wheeler and Chambers [33] suggest that it is not necessary to transform data that are used in control charts, because control charts work well in situations where data are not normally distributed. They give an excellent demonstration of the performance of control charts when data are collected from a variety of nonsymmetric distributions.
Minitab provides two Box-Cox transformations: a standalone command, described in this section, and a transformation option provided with all control charts, except the Attributes charts and Rare Event charts. You can use these procedures in tandem. First, use the standalone command as an exploratory tool to help you determine the best lambda value for the transformation. Then, when you enter the control chart command, use the transformation option to transform the data at the same time you draw the chart.
All observations for a chart are in one column: Choose if data are in one or more columns, then enter the columns.
Subgroup sizes: Enter a number or a column of subscripts.
Observations for a subgroup are in one row of columns: Choose if subgroups are arranged in rows across several columns, then enter the columns.