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Statistics for Six Sigma Green Belts with Minitab and JMP by David M. Levine

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11.6. The u-Chart

In some applications, the areas of opportunity vary in size. Generally, the construction and interpretation of control charts are easier when the area of opportunity remains constant, but sometimes changes in the area may be unavoidable. For example, samples taken from a roll of paper may need to be manually torn from rolls, so that the areas of opportunity will vary; and the number of typing errors in a document will have areas of opportunity that will vary with the lengths of the documents. When the areas vary, the control chart you use is a u-chart.

The u-chart is similar to the c-chart because it is a control chart for the count of the number of events, such as the number of defects (nonconformities over a given area of ...

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