Six Sigma - Defect Metrics


Before we go ahead, let us define two terms:

  • A Six Sigma defect is defined as anything outside of customer specifications.

  • A Six Sigma opportunity is the total quantity of chances for a defect.

This chapter provides a list formulae normally used to measure different metrics related to Six Sigma defects.

Defects Per Unit - DPU

         Total Number of Defects
DPU =    ------------------------
         Total number of Product Units

The probability of getting 'r' defects in a sample having a given DPU rate can be predicted with the Poisson Distribution.

Total Opportunities - TO

TO = Total number of Product Units x Opportunities

Defects Per Opportunity - DPO

         Total Number of Defects
DPO =    ------------------------
         Total Opportunity

Defects Per Million Opportunities - DPMO

DPMO =   DPO x 1,000,000

Defects Per Million Opportunities or DPMO can be then converted to sigma values using Yield to Sigma Conversion Table given in Six Sigma - Measure Phase.

According to the conversion table −

6 Sigma = 3.4 DPMO

How to find your Sigma Level

  • Clearly define the customer's explicit requirements.
  • Count the number of defects that occur.
  • Determine the yield-percentage of items without defects.
  • Use the conversion chart to determine DPMO and Sigma Level.

Simplified Sigma Conversion Table

If your yield is Your DPMO is Your Sigma is
30.9% 690,000 1.0
62.9% 308,000 2.0
93.3 66,800 3.0
99.4 6,210 4.0
99.98 320 5.0
99.9997 3.4 6.0