The Moving Range (MR) chart assesses whether the between-subgroup variation is in control. Each test for special causes detects points beyond the control limits and specific patterns in the data plotted on the MR chart.

·    When a subgroup fails a test, Minitab reports it in the Session window and marks it on the MR chart. Failed points indicate that there is more variation between two consecutive subgroups than expected. These subgroups should be investigated.

·    When no points are displayed under the test results, no subgroups failed the tests for special causes.

For the injection molding data, Subgroup 14 failed Test 1 on the MR chart. Test 1 tests for single points that are more than 3 s from the center line. This test provides the strongest evidence for lack of control in a process.