A zone chart is a simplified control chart that can be used to assess whether your process is in control.

A zone chart assigns weights to plotted points based on the zone, located 1, 2, or 3 s from the center line, in which it is located. Consecutive plot point weights located on the same side of the center line are summed into a cumulative score. When points cross the center line, the cumulative score is set to zero. When the cumulative score equals or exceeds 8, then the process can be considered out of control.

Zone charts can be used for data in subgroups with equal or unequal subgroup sizes, or with individual observations.

A manufacturer wants to determine whether the steel bar process is in control. The process improvement team measured the length of ten sets of steel bars for six shifts. The team decided to use a zone chart to evaluate the 60 subgroups collected.

Data: Steel.MTW (available in the Sample Data folder).