The Xbar-R chart consist of is a control chart of subgroup means and a control chart of subgroup ranges. It is used to track both the process level and process variation at the same time, and detect signs of special causes.
1. Choose ProcessMA > Control Charts > Variable Charts for Subgroups > Xbar-R.
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 Control limit multiples, enter the position to draw the upper and lower control limits. Multiple entries must be separated by commas (,).
7. In Lower limit bound, enter the bound on the lower control limit.
8. In Upper limit bound, enter the bound on the upper control limit.
9. In Phase, select the column containing the phase codes. (Phase can useful to compare groups of data like before and after improvement).
10. In Estimate, Select the method of estimating sigma.
· Rbar - Estimate based on the average of the subgroup ranges.
· Pooled standard deviation - Estimate based on the pooled standard deviation.
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.
Control limit multiples: Numeric.
Lower limit bound: Numeric.
Upper limit bound: Numeric.
Phase: Text or Numeric; Must contain equal number of data points as the Variable.
You are the branch manager of a bank and you need to ensure that time customers spent waiting in the queue is in control. For each day you took 5 measurements on the waiting time. You want to generate a Xbar-R Chart to track the process level and variation.
1. Open worksheet ProcessMA > Tools > Data Files > Control.xls.
2. Choose ProcessMA > Control Charts > Variable Charts for Subgroups > Xbar-R.
3. In Variable, select B – Waiting time.
4. In Subgroup, enter 5
5. Click OK.
Interpretation
The ninth point of the Xbar Chart lies outside of the control limit suggesting a possible special cause. The points in the R Chart is random and within the control limit, suggesting that the variation of the process is stable.