A subgroup is a group of units that are created under the same set of conditions. Subgroups (or rational subgroups) represent a "snapshot" of the process. Therefore, the measurements within a subgroup must be taken close together in time but still be independent of each other. ... Each sample of five parts is a subgroup.

Is a subgroup a sample?

Re: Subgroup Size

A Rational Subgroup is simply "a sample in which all of the items are produced under conditions in which only random effects are should be responsible for the observed variation." [Nelson, Lloyd S. "Control Charts: Rational Subgroups and Effective Applications," Journal of Quality Technology.

What's the meaning of subgroup?

1 : a subordinate group whose members usually share some common differential quality. 2 : a subset of a mathematical group that is itself a group.