XBar & Sigma
XBar & Sigma Control Chart

The XBar & Sigma chart, like the XBar & Range chart provides a graph of the average of a process characteristic. The difference is that the lower chart is a Sigma chart rather than a Range chart.

The Sigma chart indicates the standard deviation of a subgroup's values. The Sigma chart also gives a picture of the change in variability. A stable process will have a fairly uniform standard deviation, while an unstable process will exhibit significant changes. A Sigma chart should be in control before evaluating the XBar chart.