Relationship between the R Chart and X-bar Chart

If the R chart indicates that the process variability is out of control, then you should disregard the X-bar chart. The average of the sample ranges is used in the calculation of the control limits for the X-bar chart. Therefore, if the R chart does not display control, the control limits on the X-bar chart are questionable. Since the process variability appears to be in control, you can safely examine the X-bar chart.

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