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