Cpk
Also known as: Process Capability Index (short-term)
Definition
Cpk is a short-term process capability index that measures how well a process fits within its specification limits, accounting for centring; values above 1.33 are typically required for a controlled process.
Formula
Cpk = min((USL − μ) / (3σ̂_within), (μ − LSL) / (3σ̂_within))In depth
Cpk is calculated using short-term variation (within-subgroup standard deviation, σ̂_within) rather than long-term variation. It takes the minimum of two one-sided capability ratios: how close the process mean is to the upper specification limit, and how close it is to the lower.
A Cpk of 1.00 means three standard deviations fit between the process mean and the nearest spec limit — about 2,700 defects per million. A Cpk of 1.33 corresponds to ~63 DPM; 1.67 to ~0.6 DPM. IATF 16949 typically requires Cpk ≥ 1.33 for special characteristics, with 1.67 commonly demanded by automotive OEMs at PPAP.
Cpk only describes the process's potential when stable. For long-term observed performance, use Ppk.