**Cpm** is a capability index, also known as the Taguchi capability index, that is a function of the specification limits, mean of the process, and a provided target, T.

The indices of Cpk and Ppk focus on centering of the mean of the distribution around the midpoint of the tolerances (USL+LSL/2) but this is not always the customer target.

Sometimes the customer may specify targeting the upper or lower end of the tolerance. This could be due to past experiences, part weight reduction, and many other reasons.

The value of Cpm is always > 0 and assumes normally distributed process output and uses the sample standard deviation calculation for sigma (within).

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The links below explain other process capability indices.

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