Process Control subgroups are
collected from the process and their statistics
(typically the mean and range or mean and
standard deviation) are plotted on control
The subgroups should be collected in a
way that minimizes the opportunities for
causes to occur within a subgroup.
The items in a subgroup should be collected
closely together, and can even be consecutive
items. Items within a subgroup should not
be collected across a shift change, or across
any other process change that may be associated
with a special cause.