Interpretation & Calculations
A Process Capability Index. The ratio of the tolerance (specification, or permitted amount of variation) to the process variation.
A value of 1 indicates the process variation exactly equals the tolerance.
Values of less than 1 indicate the allowable variation (the tolerance) is less than the process variation.
Values of more than 1 indicate that the process variation is less than the tolerance.
Values greater than 1 are most desirable, and many customers recommend values of 1.33 or greater. This allows a margin of error in case a process shift occurs that is not immediately detected.
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).