Rational Subgroups
Process standard deviation is estimated from the variability within subgroups. A point lies outside the control limits when the variation between subgroups is large relative to the variation within subgroups. We conclude that a special cause is present. The within subgroup variation is our estimate of common cause variation. Some people prefer to call this 'short term variation' from subgroup to subgroup.
It is important that we try to choose our subgroups in such a way that common cause variability is represented in within subgroup variation, but special causes occur between subgroups. (Shewhart called this a rational choice, and coined the term 'rational subgroups' which has remained in vogue.)
It is important to think carefully about the sampling scheme, and use it to decide what variation is represented by differences within the subgroup, and what variation causes differences between subgroups. You may sample differently depending on the purposes of the study.