Pp Index

A Process Performance Index. The ratio of the tolerance (specification, or permitted amount of variation) to the variation in a sample.

A value of 1 indicates the sample variation exactly equals the tolerance.

Values of less than 1 indicate the allowable variation (the tolerance) is less than the sample variation.

Values of more than 1 indicate that the sample variation is less than the tolerance.

Use of Performance Indices is generally discouraged in favor of Capability Indices wherever possible.

Normal distributions:

Johnson distributions:

where:

σx is Sample Sigma .

Ordinate0.99865 and ordinate0.00135 are the z-values of the non-normal cumulative distribution curve at the 99.865 percentage point and the 0.135 percentage points, respectively.

See also:

Process Capability

Ppk

Pr

Ppm

Process Performance

Learn more about the SPC principles and tools for process improvement in Statistical Process Control Demystified (2011, McGraw-Hill) by Paul Keller, in his online SPC Concepts short course (only $39), or his online SPC certification course ($350) or online Green Belt certification course ($499).