sigma level

**Sigma Level** is the **number of standard deviations between the process mean and the nearest specification limit** — a higher sigma level means a more capable process with fewer defects, directly relating to the expected defect rate in parts per million. **Sigma Level to DPPM Mapping** - **3σ**: 66,807 DPPM (93.32% yield) — barely acceptable for most applications. - **4σ**: 6,210 DPPM (99.38% yield) — typical starting capability. - **5σ**: 233 DPPM (99.977% yield) — high capability. - **6σ**: 3.4 DPPM (99.99966% yield) — world-class quality (with 1.5σ shift). **Why It Matters** - **Universal Metric**: Sigma level provides a common language for comparing process capability across different parameters and processes. - **Goal Setting**: Sigma level targets drive improvement programs — "move from 4σ to 5σ" is a clear objective. - **1.5σ Shift**: The conventional 6σ = 3.4 DPPM includes an assumed 1.5σ long-term mean shift — without shift, 6σ = 0.002 DPPM. **Sigma Level** is **the capability number** — a universal measure of process quality that translates directly to expected defect rates.