Process Capability Index (Cpk)
Definition
The key measure of actual process capability, calculated as Cpk = min(Cpl, Cpu) where Cpl = (Mean - LSL)/(3σ) and Cpu = (USL - Mean)/(3σ). Unlike Cp, Cpk accounts for process centering.
Practical Note
Cpk ≥ 1.33 required for existing processes, ≥ 1.67 for new/critical processes. Cpk = 1.0 means 0.27% defects (2700 PPM). Cpk = 2.0 corresponds to Six Sigma quality (3.4 PPM).