The Zone chart assesses whether the process is in control. Each time the points cross the center line, the cumulative score is set to zero. Each cumulative score helps determine whether or not the process is in control. When the cumulative score equals or exceeds 8, the process is out-of-control.

·    When a subgroup's cumulative score exceeds 8, Minitab reports it in the Session window and marks it on the Zone chart. A failed subgroup indicates that there is a nonrandom pattern in the data that may be the result of special-cause variation. These subgroups should be investigated.

·    When no points are displayed in the Session window, then no subgroup's cumulative score exceeded 8.

For the steel bar process data, subgroups 5 and 10 exceeded a cumulative score of 8.