Defect Metrics - Six Sigma Concept

Lets define two terms before going forward:
  • In Six Sigma, a defect is defined as anything outside of customer specifications.
  • In Six Sigma, an opportunity is the total quantity of chances for a defect.
This article provides a list formula normally used to measure different metrics related to Six Sigma defects.

Defects Per Unit (DPU):           

DPU = (total number of defects)/(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):

DPO = (t
otal number of defects)/ (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

     Yield                  DPMO                Sigma
       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

Six Sigma - Summary
  • Six Sigma is a philosophy of quality improvement.
  • Six Sigma is 3.4 defects in one million opportunities (DPMO).
  • Components of Six Sigma are Customer, Process, and Employees.
  • Six Sigma implementation requires the following roles:
    • Business Leader
    • Sponsor
    • Black Belt
    • Master Black Belt
    • Green Belt
  • The generic cycle of Six Sigma includes the following phases: Define, Measure, Analyze, Improve, and Control.
  • Six Sigma is dedicated to 'Customer focus'.