The Zone Chart is similar to the Xbar or individuals Chart. It plots a cumulative score based on zones. It is useful because ‘out-of-control’ is easy to identify, which is when cumulative score is greater than 8.
1. Choose ProcessMA > Control Charts > Variable Charts for Subgroups > Zone.
2. In Variable, select the column containing the data.
· For constant subgroup size:
- Check Constant size.
- In Subgroup, enter the subgroup size.
· For non-constant subgroup size:
- Uncheck Constant size.
- In Subgroup, select the column containing the subgroup codes.
3. Click OK.
Optional
4. In Mean, enter the population mean, otherwise it will be estimated by the sample mean.
5. In Standard deviation, enter the population standard deviation, otherwise it will be estimated from the data.
6. In Phase, select the column containing the phase codes. (Phase can useful to compare groups of data like before and after improvement).
7. In Method (Subgroup size=1), select the method of estimating sigma when subgroup size is 1.
· Average moving range – Estimate based on the average of the moving range.
· Median moving range – Estimate based on the median of the moving range
8. In Method (Subgroup size>1), select the method of estimating sigma when subgroup size is greater than 1.
· Rbar - Estimate based on the average of the subgroup ranges.
· Sbar -Estimate based on the average of the subgroup standard deviations.
· Pooled standard deviation - Estimate based on pooled standard deviation.
9. In Moving range length, enter the number of observations used to calculate the moving range.
10. In Zone 1 (Stddev < 1), enter the weights you want to assign to points in Zone 1.
11. In Zone 2 (Stddev < 2), enter the weights you want to assign to points in Zone 2.
12. In Zone 3 (Stddev < 3), enter the weights you want to assign to points in Zone 3.
13. In Zone 4 (Stddev > 3), enter the weights you want to assign to points in Zone 4.
14. Check Reset score after each signal, if you want to reset the cumulative score to zero following each out of control signal.
Note To select a column of data into a textbox, double-click on any of the column names shown in the list on the left of the dialog box while in the textbox.
Variable: Numeric.
Subgroup (For non-constant subgroup size): Text or Numeric; Must contain equal number of data points as the Variable.
Subgroup (For constant subgroup size): Integer; Must be at least 2.
Mean: Numeric.
Standard deviation: Numeric.
Moving range length: Integer.
Zone 1 (Stddev < 1): Numeric; Greater than 0.
Zone 2 (Stddev < 2): Numeric; Greater than 0.
Zone 3 (Stddev < 3): Numeric; Greater than 0.
Zone 4 (Stddev > 3): Numeric; Greater than 0.
Phase: Text or Numeric; Must contain equal number of data points as the Variable.
You operate a delivery service and you want to ensure that parcels are delivered to your customers on time. You recorded the delivery times for 5 parcels to the same area for the past 10 days. You want to run the Zone Chart to evaluate the situation.
1. Open worksheet ProcessMA > Tools > Data Files > Control.xls.
2. Choose ProcessMA > Control Charts > Variable Charts for Subgroups > Zone.
3. In Variable, select E – Delivery time.
4. Check Constant size.
5. In Subgroup, enter 5
6. Click OK.
Interpretation
Several points have cumulative scores equals or greater than 8 which suggest the process is out of control.