There are two types of variation in a process: within-subgroup variation and between-subgroup variation. To improve process quality, try to eliminate between-subgroup variation and reduce within-subgroup variation.
- Within-subgroup variation
- The variation between measurements within subgroups. The within-subgroup variation is used to calculate the control limits of your control chart. When you collect samples for control charts, you should select logical subgroups so that only the common-cause variation is reflected in each subgroup.
- Between-subgroup variation
- The variation that exists between subgroups that may be caused by specific identifiable factors, or special causes.
Note
For some variables control charts, extended control limits can use within-subgroup and between-subgroup variation to account for allowable changes in the process mean.
Example
This individual value plot displays measurements taken from a die cut machine. Each vertical line of symbols represents the measurements for one subgroup. The interval bars represent the within-subgroup variation. The line that connects the means of the consecutive subgroups represents the between-subgroup variation.
