Subgroup

A group of units produced under the same set of conditions. Subgroups (or rational subgroups) are meant to represent a "snapshot" of the process. Therefore, they must be taken close together in time but still be independent of each other. For example, a die cut machine produces 100 plastic parts per hour. The quality engineer measures five randomly selected parts at the beginning of every hour. Each sample of five parts is a subgroup.

Use subgroups to separate the two types of variation in a process:

·    Within subgroup: the variation among measurements within subgroups; also known as common cause variation.

·    Between subgroup: the variation between subgroups that may be caused by specific identifiable factors, or special causes.

This individual value plot displays the values of the samples taken from the die cut machine. Each vertical line of gray symbols represents values in a subgroup. The blue intervals represent the within subgroup variation, and the red mean connect line represents the between subgroup variation.

To improve process quality, you should make an effort to eliminate between subgroup variation and reduce within subgroup variation.