Interpretation & Calculations
The process capability index calculation is a measure similar to the Cpk index that also takes into account variation between the process average and a target value. If the process average and the target are the same value, Cpm will be the same as Cpk. If the average drifts from the target value, Cpm will be less than Cpk. To calculate the Cpm capability index, use the Cpm formula below, or have your SPC software calculate it for you.
where Cp is the classic capability index, T is the process target, 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.
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).