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.
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.
The variation that exists between subgroups that may be caused by specific identifiable factors, or special causes.