cp

**Cp** (Process Capability Index) is the **ratio of the specification width to the process width** — $Cp = frac{USL - LSL}{6sigma}$, 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.