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.