Interpretation & Calculations
The confidence limits for the Capability Index Cpk may be calculated as follows:
where n is the subgroup size.
This calculated value may be added and subtracted from the calculated Cpk index to indicate the range of values for Cpk 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).