Ppk (Long-Term, Centred)
Long-term counterpart to Cpk — uses overall variation and accounts for mean location.
Also known as: Ppk value, long-term Cpk
Ppk is the most realistic customer-facing metric of the capability family: it combines the overall variation of Pp with the centring of Cpk.
- Ppk ≤ Cpk as a rule
- Ppk << Cpk signals strong drift or special causes — the process can do it, but in daily life it does not
- Ppk as a reporting metric is often required in supplier audits because it mirrors actual behaviour
See also
Used in
In the Algorithm Lab
Sources
- AIAG — Statistical Process Control (SPC) Reference Manual, 2nd Edition
- Montgomery, D. C. — Introduction to Statistical Quality Control, 8th Edition, Chapter 8