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Average
Range
The average of the subgroup
ranges (R-bar).
(n = constant) , OR
(n 
constant)
where:
Note: When control
limits for the X-Bar chart are defined as fixed values (such as when historical
data is used to define control limits), the Average Range (R-bar) must be
back calculated from these pre-defined control limits. This ensures that
the control limits on the Range chart are at the same sensitivity as those
on the X-Bar chart. In this case:
where d2 (available
in any statistical quality control textbook) is based on the subgroup size
n.
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