Cpk index

A Process Capability Index. A measure of both process dispersion and its centering about the average.

where

Normal Distributions:

Non-Normal Distributions:

where x-doublebar is the Grand Average and σx is Process Sigma, which is calculated using the Moving Range (when subgroup size = 1) or Subgroup Sigma (subgroup size >1) statistic.

Znormal,p and Znormal,1-p are the z-values of the normal cumulative distribution curve at the p percentage point and the 1-p percentage points, respectively.

See also:

Process Capability

Confidence Intervals for Cpk

Capability index Calculation in Minitab

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).