Ppk
Also known as: Process Performance Index (long-term)
Definition
Ppk is a long-term process performance index that measures how well a process has actually performed against its specification limits over an extended period, using overall standard deviation rather than within-subgroup variation.
Formula
Ppk = min((USL − μ) / (3σ̂_overall), (μ − LSL) / (3σ̂_overall))In depth
Where Cpk asks 'how capable is the process when it's stable?', Ppk asks 'how has the process actually behaved over time, drift and all?'. The formula is identical except for the standard deviation: Ppk uses σ̂_overall (computed from the full dataset), which absorbs both within-subgroup variation and between-subgroup shifts.
For a perfectly stable process, Ppk ≈ Cpk. When Cpk exceeds Ppk by a wide margin, the process drifts between subgroups — investigate special causes, tooling wear, or operator changeovers.
PPAP submissions to automotive OEMs typically require Ppk ≥ 1.67 for special characteristics on the initial production run.