Cpk
process capability index
Cpk = min(USL − μ, μ − LSL) / (3 · σ_within)
Cpk measures how many three-sigma widths fit between the process mean and the nearest specification limit, using the short-term (within-subgroup) sigma. It accounts for both spread and centring.
Common requirements are Cpk ≥ 1.33 for running processes and 1.67 for new or critical ones. Cpk 1.33 corresponds to about 66 defective parts per million for a centred normal process.
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.