Steps
- Determine the number of units, the number of defect opportunities per unit and the number of defects
- Calculate DPMO
- Look up the corresponding sigma level in a sigma table
Anchor values
- 3σ ≈ 66,807 DPMO
- 4σ ≈ 6,210 DPMO
- 5σ ≈ 233 DPMO
- 6σ ≈ 3.4 DPMO
Worked example (illustrative)
10,000 units, 1 defect opportunity per unit, 68 defects → DPMO = (68 ÷ 10,000) × 1,000,000 = 6,800 → approximately 4σ.
Why this matters
Sigma level is a common language for quality performance and a widely asked-about concept. VDS measures defects and units reliably per line and per batch; you do the calculating and the improving.
Frequently asked questions
What is DPMO?
Defects Per Million Opportunities: the number of defects converted to a million defect opportunities, so that processes become comparable.
What is a good sigma level?
Higher is better; 6σ (3.4 DPMO) counts as excellent, and many processes sit around 3 to 4σ.
What is the difference between sigma and Cpk?
Sigma level looks at defects relative to the opportunity for them; Cpk looks at how well a process fits within the specification limits.
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