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:
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.