Between/Within Charts Help
The between/within charts include the following:
There are times when the “within subgroup” variation is significantly different from the “between subgroup” variation. When this occurs, the control limits the Xbar chart in a Xbar-R combination are very tight and many of the points are out of control. For example, you might have a batch process where you sample a batch four times and use those four samples to form a subgroup. The range between those four subgroups is very small. Using that average range to calculate the control limits for the Xbar chart is what causes the control limits to be so tight. The solution for this type of problem is to use three charts: Xbar-mR-R or the Xbar-mR-s.
With the Xbar-mR-R chart, the subgroup averages are plotted on the Xbar chart. The mR is used to monitor the variation in the range between consecutive subgroup averages. The R chart is used to monitor the variation in the within subgroup range. The Xbar-mR-s chart is essentially the same but the s (subgroup standard deviation) chart is used in place of the R chart.
These are accessed by selecting “Variable” from the “Control Charts” panel on the SPC for Excel ribbon.
Select the links above for help on making each of these charts.