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Confidence
Limits for Capability Index Cpm
The confidence limits for
the Capability Index Cpm may be calculated
as follows:
where n is the subgroup size, x-doublebar is the Grand
Average and  x
is Process Sigma, which is calculated using the Moving
Range (when subgroup size = 1) or Subgroup
Sigma (subgroup size >1) statistic.
This calculated value
may be added and subtracted from the calculated Cpm index to indicate the
range of values for Cpm expected from random samples of a stable process.
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