Sigma level is a single-number summary of process quality that rises as DPMO falls. It comes from asking how many standard deviations fit between the process average and the nearest specification limit, translated into a defect rate. For a beginner the useful part is the reference table, not the derivation. A few approximate anchor points: about 66,800 DPMO corresponds to 3σ, about 6,210 DPMO to 4σ, about 233 DPMO to 5σ, and 3.4 DPMO to 6σ.
Those figures assume a conventional 1.5 sigma shift, meaning the published table allows for the process average drifting 1.5 standard deviations over the long term, so a process rated 6σ short-term delivers 3.4 DPMO rather than a far smaller number. You do not need to agree with the convention, but you do need to know your table uses it so you compare like with like.
Between the anchor points, interpolate roughly or use a fuller lookup table. The scale is not linear: going from 4σ to 5σ cuts defects by a factor of about 27, while going from 5σ to 6σ cuts them by a factor of about 70. Small gains in sigma level at the top end represent enormous improvements in defect rate.
Standard Procedure
- 1Calculate DPMO as in Lesson 3.
- 2Locate the DPMO between the nearest anchor points on the lookup.
- 3Read the approximate sigma level, interpolating or using a fuller table.
- 4Report the DPMO alongside the sigma level and state that the 1.5 shift convention applies.
Lesson Summary
- ✓Sigma level is a lookup from DPMO, not a separate measurement.
- ✓The scale is steep at the top: each extra sigma cuts defects by a large multiple.