Pp and Ppk are process performance indices. They evaluate the actual output of the process over a specific period.
Difference from Cp and Cpk
Cp and Cpk are capability indices and are built on the condition that the process is in statistical control, with only common causes acting. They are based on short-term variation.
Pp and Ppk use the total variation of the period, including variation from special causes. They therefore usually come out lower than the capability indices.
Which one when?
Performance indices are used when a new process is being introduced or when the process has not yet been shown to be in control. Capability indices are meaningful once the process has become stable.
Which index is requested in PPAP submissions depends on customer requirements; performance indices are mostly requested in initial submissions.
Common mistake
Using the two index sets interchangeably or not stating which one is reported. A large difference between the two is a sign that the process is not in control.
The correct use and reporting of the two index sets is addressed in the SPC training. For their presentation in the PPAP package, see the PPAP training.