Six Sigma - Defect Metrics


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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


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