Six Sigma - Defect Metrics

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

Six Sigma - Summary

We can summarize the following points:

●      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'.