Confidence Limits for Capability Index Cpm

The confidence limits for the Capability Index Cpm may be calculated as follows:

where

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

This calculated value may be added and subtracted from the calculated Cpm index to indicate the range of values for Cpm expected from random samples of a stable process.

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).