Example of Zone Chart

A quality engineer wants to determine whether a steel bar cutting process is in control. The engineer measures the length of five steel bars for ten shifts.

The engineer creates a zone chart to monitor the length of steel bars.

  1. Open the sample data, SteelBarLength.MTW.
  2. Choose Stat > Control Charts > Variables Charts for Subgroups > Zone.
  3. From the drop-down list, select All observations for a chart are in one column, enter Length.
  4. In Subgroup sizes, enter Subgroup ID.
  5. Click Zone Options, then select the Weights/Reset tab.
  6. Check Reset cumulative score after each signal.
    If you investigate and correct your process when it is out-of-control, it is appropriate to reset the cumulative score.
  7. Click OK in each dialog box.

Interpret the results

For the steel bar length data, subgroup 5 has a cumulative score of 8, and subgroup 10 has a cumulative score of 8. The manufacturing process is out of control.

Zone Chart of Length

Test Results for Zone Chart of Length

TEST. Cumulative score greater than or equal to zone 4 score. Test Failed at points: 5, 10 * WARNING * If graph is updated with new data, the results above may no longer be correct.