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.
At the Excel Menu (For Excel 2007, go to Add-ins first)
Choose ProcessMA > Control Charts > Variables Charts for Subgroups > Zone
In Variable, select the column containing the data (Numeric)
In Subgroup, select the column containing subgroup indicators (Optional, >=2 distinct values)
In Constant Subgroup Size, enter the subgroup size if it is constant (Optional, Integer, >=2)
In Historical Mean, enter the population proportion, otherwise it will be estimated from the sample (Optional, Numeric)
In Historical Standard Deviation, enter the population standard deviation, otherwise it will be estimated from the data (Optional, Numeric, >0)
In Control Limit Multiples, enter the position to draw the upper and lower control limits. Multiple entries must be separated by commas (Optional, Numeric, >0, Up to 3 multiples)
In Lower Limit Bound, enter the bound on the lower control limit (Optional, Numeric)
In Upper Limit Bound, enter the bound on the upper control limit (Optional, Numeric)
In Phase, select the column containing the phase indicators (Optional, >=2 distinct values)
In Method (Subgroup>1), choose the method of estimating sigma when subgroup size is greater than 1
In Method (Subgroup=1), choose the method of estimating sigma when subgroup size is equals to 1
In Moving Range Length, enter the number of observations used to calculate the moving range (Integer, >=2)
In Zone 1 (Std Dev <1), enter the weights you want to assign to points in Zone 1
In Zone 2 (Std Dev <2), enter the weights you want to assign to points in Zone 2
In Zone 3 (Std Dev <3), enter the weights you want to assign to points in Zone 3
In Zone 4 (Std Dev >3), enter the weights you want to assign to points in Zone 4
Check Reset Score After Each Signal, if you want to reset the cumulative score to zero following each out of control signal
Click OK
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.
Open data worksheet by choosing ProcessMA > Tools > Data
Choose ProcessMA > Control Charts > Variables Charts for Subgroups > Zone
In Variable, select AB - Delivery Time
In Constant Subgroup Size, enter 5
Several points have cumulative scores equals or greater than 8 which suggest the process is out of control.
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