At Six Sigma, what does the level mean in terms of defects per million?

• A. 99.999% of manufactured items are within the specification (or only 3.4 defects per million)
• B. 95% within specification (50,000 defects per million)
• C. 99% within specification (10,000 defects per million)
• D. 100% zero defects

Answer: (A)

Six Sigma measures quality by defects per million opportunities (DPMO). At this level, the aim is about 3.4 defects per million opportunities, which corresponds to roughly 99.99966% of items meeting the specification. The 3.4 DPMO result comes from accounting for a 1.5 sigma shift in long-term process performance, so the stated interpretation—only a handful of defects per million and almost all items within spec—is the essence of Six Sigma.

Other descriptions with higher defect rates, like 95% or 99% within specification, imply far more defects per million and don’t match the Six Sigma target. Zero defects is an ideal not typically achievable in practice, whereas Six Sigma targets a near-perfection level reflected by about 3.4 defects per million.