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P<sub>p

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Definition

A measure of process performance relative to its tolerance. It is calculated as the ratio of the tolerance to the six sigma spread. The formula for Pp is based on the calculated sigma (whereas Cp uses the estimated sigma) and does not take into account the centering of the process (it assumes the process is centered).

Examples

The picture alongside shows the formula to calculate Pp, where USL and LSL are the upper and lower specification limits respectively and 'scalculated' is the process standard deviation calculated directly from the data.

Consider an example:

The Lower Specification Limit is 48
The Upper Specification Limit is 60
Therefore, the Tolerance is 60 - 48, or 12
The calculated standard deviation is 1.65, giving a 6-Sigma process spread of 9.9.
Therefore, the Pp is 12/9.9 or 1.212.

Application

Pp and the related index Ppk are used when it is necessary to understand the current performance of the process relative to its specifications. For the long term potential of the rpocess to meet these requirements, you should use the capability index Cp.

See Also

Ppk, Cp, Cpk