A zone chart assess stability using scores that are based on the distance of the subgroup means from the center line. The "zones" are at 1, 2, and 3 standard deviations from the center line. It is a hybrid between an Xbar (or individuals) control chart and a CUSUM control chart. A point is out of control by default if its cumulative score is greater than or equal to 8 (the zone 4 score). Use zone charts when you have variables data and want to create an easily interpretable control chart.
By default, the weights for each zone are:
The appeal of zone charts is their simplicity. A point is out of control, by default, if the cumulative score for the point is greater than or equal to 8. Thus, you do not need to recognize the patterns associated with the tests on an I-MR, Xbar-R or Xbar-S chart. This method is equivalent to four of the standard tests for special causes.
A manufacturer wants to assess the length of a part. The quality team samples 10 parts from each of 3 shifts for 4 days.
The first subgroup has a score of 0 because this point falls in zone 1. Subgroup 2 has a zone score of 4 because it falls in zone 3. The zone scores are added for consecutive points to calculated the cumulative score. Each point is labeled with its cumulative score. By default, the cumulative score is reset to zero when consecutive points are on opposite sides of the center line. For example, subgroup 6 has a cumulative score of zero. Subgroup 7 has a zone score of 8, indicating an out-of-control situation.