The R chart assesses whether the within-subgroup variation is in control. An R chart consists of the following:

·    Plotted points, which represent the subgroup ranges, provide an estimate of the variation within subgroups.

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

·    Control limits (red), which are 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 printed 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 for the injection molding data can be summarized as follows:

·    The R chart's lower and upper control limits are 0 and 1.196, respectively. Therefore, the subgroup ranges are expected to fall between 0 and 1.196. The center line (average moving range) is 0.566.

·    One subgroup failed Test 1 on the R chart. Test 1 tests for single points that are more than 3 s from the center line and provides the strongest evidence for lack of control in a process.