Module 3 · Lesson 15 of 17
Sigma-level interpretation and limitations
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Learning objectives
- Convert yield / DPMO to a sigma level and back.
- Explain the 1.5σ shift as a modelling convention, not a law.
- Communicate performance without false precision.
What sigma level is
Sigma level standardises defect probability for communication. It is derived from yield or DPMO under a normal-distribution model, often with a 1.5σ long-term shift assumption.
It should always be reported alongside the underlying DPMO or yield so the audience sees the raw performance.
Garment-factory example
A cutting operation at 20,000 DPMO reads roughly 3.6σ short-term or 5.1σ with the 1.5σ shift; the honest reporting shows DPMO plus the shift assumption used.
Method
- State the calculation method and shift convention.
- Convert with a free reference table or spreadsheet.
- Report raw yield/DPMO alongside the sigma value.
- Do not compare sigma levels across incomparable opportunity definitions.
Common mistakes
- Claiming ‘world-class’ from inflated opportunity counts.
- Comparing measurement-process sigma with inspection-defect sigma.
Knowledge check
Pick one answer per question. Explanations appear after you submit.
1. The 1.5σ shift is:
2. Sigma level should be reported with:
Author: NOSK.life · Last reviewed: 2026-07-21