The XBar and Range provides a graph of the average of a process characteristic
and the range extent. The XBar chart gives a picture of how well a process is
centered and if it is stable. XBar is the average of the set of subgroup
values. If subgroup points fall outside of the limits or if identifiable trends
exist, this indicates a change is occurring in the process which will most
likely result in increased variability.
The Range chart indicates the range of a subgroup's values. The Range chart
gives a picture of the change in variability. A stable process will have a
fairly uniform range, while an unstable process will exhibit significant
changes in the range. A Range chart should show control before evaluating the
XBar chart.