Interpretation & Calculations
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.
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.
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).