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XBar and 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.
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