Six Sigma - Defect Metrics


Before we go ahead, lets 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.

Here are various formulae 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