Confidence Limits for Capability Index Cpm

The confidence limits for the Capability Index Cpm may be calculated as follows:

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where

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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.