Question 1

Why is a 1.5 sigma shift applied in Six Sigma calculations?

Accepted Answer

The 1.5σ shift was introduced by Motorola to account for long-term process drift and variation that naturally occurs over time. Short-term studies may show better capability, but real-world processes shift. This convention means a '6 Sigma' process actually has 4.5σ between the mean and spec limit long-term, yielding 3.4 DPMO rather than 0.002 DPMO.

Question 2

What is the difference between PPM and DPMO?

Accepted Answer

PPM counts defective units per million units produced, while DPMO (Defects Per Million Opportunities) counts defects per million opportunities for a defect. A single unit may have multiple opportunities for defects. DPMO = (Defects / (Units × Opportunities per Unit)) × 1,000,000. DPMO is more granular and is preferred when products have multiple quality characteristics.