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X-Bar and R Charts: This set of two charts is the most commonly used statistical process control procedure. Used to monitor process behavior and outcome overtime.
X-Bar and R charts draw a control chart for subgroup means and a control chart for subgroup ranges in one graphic. Interpreting both charts together allows you to track both process center and process variation and detect the presence of special causes. Generally, a user focuses on the range portion of the chart first, confirming that the process is in control. Finally, the user focuses on the average chart, looking for special cause there.