The Xbar chart plots the average of the measurements within each subgroup. No points are out of control on the R chart (the bottom chart).
Xbar and r bar charts how to#
For more information, go to Specify how to estimate the parameters for Xbar-R Chart. If out-of-control points are due to special causes, then consider omitting these points from the calculations. Out-of-control points can influence the estimates of process parameters and prevent control limits from truly representing your process. If the chart shows out-of-control points, investigate those points. If the same point fails multiple tests, then the point is labeled with the lowest test number to avoid cluttering the graph. Red points indicate subgroups that fail at least one of the tests for special causes and are not in control. The control limits on the R chart, which are set at a distance of 3 standard deviations above and below the center line, show the amount of variation that is expected in the subgroup ranges. If the subgroup sizes differ, then the value of the center line depends on the subgroup size, because larger subgroups tend to have larger ranges.
If the subgroup size is constant, then the center line on the R chart is the average of the subgroup ranges. If the R chart is not in control, then the control limits on the Xbar chart are not accurate. Before you interpret the Xbar chart, examine the R chart to determine whether the process variation is in control.
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