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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% |
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