Cpk index

A Process Capability Index. A measure of both process dispersion and its centering about the average.

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where

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Normal Distributions:

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Non-Normal Distributions:

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where x-doublebar is the Grand Average and σx is Process Sigma, which is calculated using the Moving Range (when subgroup size = 1) or Subgroup Sigma (subgroup size >1) statistic.

Znormal,p and Znormal,1-p are the z-values of the normal cumulative distribution curve at the p percentage point and the 1-p percentage points, respectively.