Cp

Cp (Process Capability Index) is the ratio of the specification width to the process width — $Cp = \frac{USL - LSL}{6\sigma}$, measuring the inherent capability of the process to fit within the specification range, WITHOUT considering how well the process is centered.

Cp Interpretation

• Cp = 1.0: Process spread exactly equals the specification range — 2,700 PPM defect rate (if centered).
• Cp = 1.33: Process fits within 75% of the spec range — 63 PPM (standard minimum requirement).
• Cp = 1.67: Process fits within 60% of the spec range — 0.6 PPM (automotive requirement).
• Cp = 2.0: Process fits within 50% of the spec range — 0.002 PPM (Six Sigma level).

Why It Matters

• Potential: Cp measures the POTENTIAL capability — the best the process can achieve if perfectly centered.
• Centering Blind: Cp does not account for process centering — a process can have high Cp but produce many defects if off-center.
• Baseline: Cp establishes the theoretical capability — Cpk shows the realized capability including centering.

Cp is the width test — measuring whether the process variation is narrow enough to fit within the specification range, regardless of where it's centered.