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Calculation of Cpk, CPL, and CPU for Non-Normal Data

The distribution generic formulas for Cpk, Cp, CPL, and CPU are as follows:

Non-Normal Cpk Formulas

Where Px is the xth percentile of the distribution. For example, for the Normal distribution, P.5 is the mean.

Note

When the distribution is normal, the parameters simplify to the following:

  • P.5 = Mean
  • P.00135 = Mean - 3*sigma
  • P.99865 = Mean + 3*sigma

Using these relationships, it is easy to show equivalence to the equations typically used for Cpk. For example, for Cp:

Cpk Equivalence