R charts assess whether the process variation is in control. An R chart consists of the following:

·    Plotted points, which represent the subgroup ranges.

·    Center line (green), which is the estimate of process variation (the average of all the subgroup ranges).

·    Control limits (red), located above and below the center line, provide a visual means for assessing when the process is out of control. The limits, which represent the amount of variation expected for the subgroup ranges, are calculated using the variation within subgroups.

Minitab conducts up to four tests for special causes for the R chart, which detect points beyond the control limits and specific patterns in the data. Points that fail are marked with a red symbol and the number of the failed test. Complete results are available in the Session window. A failed point indicates that there is a nonrandom pattern in the data that may be the result of special-cause variation. These points should be investigated.

The R chart must be in control before you can interpret the chart. If the R chart is not in control then the control limits for the chart will be inaccurate and may inappropriately signal an out of control condition on the chart

The R chart for the camshaft data can be summarized as follows:

·    The lower and upper control limits are 0 and 5.751, respectively. Therefore, the subgroup ranges are expected to fall between 0 and 5.751. The center line (estimate of process variation) is 2.72.

·    None of the subgroup ranges are outside the control limits. Furthermore, the points inside the limits display a random pattern. This R chart does not provide any evidence for lack of control. Thus, the process variation is in control and it is appropriate to examine the chart to determine whether or not the process center is in control.