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.
|1||within 1 standard deviation of center line||0|
|2||between 1 and 2 standard deviations||2|
|3||between 2 and 3 standard deviations||4|
|4||greater than 3 standard deviations||8|
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.