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: