Ppk
process performance index
Ppk = min(USL − μ, μ − LSL) / (3 · σ_overall)
Ppk is like Cpk but uses the overall standard deviation. The AIAG PPAP manual uses Ppk for the initial process study with an acceptance level of 1.67.
A large gap between Cpk and Ppk means the process moves between subgroups.
Calculate it
Process Capability & Control Charts: Cp, Cpk, Pp, Ppk, PPM and confidence intervals with X-bar/R, X-bar/S or I-MR charts and all 8 Nelson rules.