Range Chart

Examine the R chart first. The R chart uses sample ranges to monitor changes in the spread of a process.

The sample ranges are plotted as points on the chart and connected with lines. The solid centerline is the average of the sample ranges. The two dashed lines are the upper control limit (UCL) and the lower control limit (LCL). Note that the value of the LCL is 0.0, so it's not visible because it's flush with the horizontal axis. The control limits are calculated so that if the process is in control, nearly all of the sample points will fall between the limits. Control limits reflect the expected amount of variation in the sample ranges when only common causes of variation are present.

This R chart seems okay. All the points are within the control limits. There are no obvious patterns. At the right end of the chart, there may be the beginnings of a problem with too little variability, but this is difficult to assess without further observation.

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