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 P_{p} is based on the calculated sigma (whereas C_{p} 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 P_{p}, where USL and LSL are the upper and lower specification limits respectively and 's_{calculated}' 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 P_{p} is 12/9.9 or 1.212.

Application

P_{p} and the related index P_{pk} 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 C_{p}.

See Also

P_{pk},
C_{p},
C_{pk}