XBar-S chart
properties | Configuring an XBar-S
chart | XBar-S chart
calculations

The XBar-S chart displays two charts: a control chart for
subgroup means (XBar chart) and a control chart for subgroup
standard deviations (Sigma chart) that are drawn in the same graph
window in the upper and lower halves of the screen respectively.
Standard deviation uses each individual reading to calculate
variability; therefore, Sigma is a more efficient indicator of
process variability than Range, especially with larger sample
sizes. This composite chart helps to track both the process level
and process variation at the same time, as well as to detect the
presence of special causes.

In the following XBar-S chart example, the subgroup size for the
sample data is 5. In the top chart (XBar), the mean (average) of
each subgroup is plotted. In the bottom chart (Sigma), the sigma
(standard deviation) of each subgroup is plotted. The control
limits (shown on the chart as UCL, CL, LCL, WL1, and WL2) are set
according to the data and a sigma limit of 3.

### See Also