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Six Sigma Conversion Tables
Two tables below provide both short-term and long-term conversions of Sigma Level (Z-Score), CPK or PPK, and Defect Rate respectively. Notice the difference between short-term and long-term is an industry standard 1.5 shift of the Sigma Level.

Short-Term


Sigma Level CPK Defect Rate
0.0 0.00 50.0000%
0.5 0.17 30.8538%
1.0 0.33 15.8655%
1.5 0.50 6.6807%
2.0 0.67 2.2750%
2.5 0.83 0.6210%
3.0 1.00 0.1350%
3.5 1.17 0.0233%
4.0 1.33 0.0032%
4.5 1.50 0.0003%
5.0 1.67 0.0000%
5.5 1.83 0.0000%
6.0 2.00 0.0000%


Many practitioners will be more interested in long-term defect rates as these are commonly used in cost-benefit analysis by management making critical business decisions invovling how to invest capital and so forth. Notice the margin of diminishing return between 3 and 4 Sigma. Many companies might be willing to run with scrap rates around 0.62%. In-fact the cost of making improvements beyond a 4 Sigma level may not be worth the investment. That's why it's more common for aerospace and sometimes automotive companies to push 5 and 6 Sigma levels but only in cases where the cost of product failure justifies that level of process control.

Long-Term


Sigma Level PPK Defect Rate
1.5 0.50 50.0000%
2.0 0.67 30.8538%
2.5 0.83 15.8655%
3.0 1.00 6.6807%
3.5 1.17 2.2750%
4.0 1.33 0.6210%
4.5 1.50 0.1350%
5.0 1.67 0.0233%
5.5 1.83 0.0032%
6.0 2.00 0.0003%