Cpk (Process Capability Index) quantifies how well a process output stays within specification limits relative to its natural variation, measuring both centering and spread. Definition: Cpk = min[(USL - mean) / (3sigma), (mean - LSL) / (3sigma)]. Takes the worse of upper and lower capability. Interpretation: Cpk = 1.0: Process spread equals spec width, 0.27% out-of-spec (2700 ppm). Cpk = 1.33: Standard minimum for production processes (~63 ppm). Cpk = 1.67: High-capability process (~0.6 ppm). Cpk = 2.0: Six-sigma capable process (~0.002 ppm). Cp vs Cpk: Cp measures spread only (ignores centering). Cpk accounts for process mean offset from specification center. Cpk <= Cp always. Cpk < 1.0: Process is not capable - significant fraction of output falls outside specifications. Immediate improvement required. Requirements: Semiconductor fabs typically require Cpk >= 1.33 for established processes, >= 1.67 for critical parameters. Calculation requirements: Need sufficient data (typically 25+ subgroups). Process must be in statistical control (stable). Normal distribution assumed. SPC relationship: Cpk calculated from SPC data. Ongoing monitoring ensures process remains capable. Improvement: Increase Cpk by reducing variation (sigma) or centering process on target. Short-term vs long-term: Cpk from short sample may overstate capability. Ppk uses long-term data and is often more realistic. Reporting: Cpk reported for key process parameters in qualification reports, process reviews, and customer quality reports.