Cpk or Ppk??
Jacques-Alain TIAKO, Manager, France, Member
Cpk - It's the capability index for a stable process (the process must be in a state of statistical control). The sigma calculation is based on within subgroup variation.
Ppk - It's the performance index (the actual process PERFORMANCE). The estimate of sigma is based on total variation. Ppk must be calculated if less than 100 samples or when the process is chronically unstable but meeting the specifications and in a predictable pattern.