Design of Experiments Guide • Prospective Sample Size and Power • Sigma Quality Level
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Sigma Quality Level
The Sigma Quality Level feature is a simple statistic that puts a given defect rate on a “six-sigma” scale. For example, on a scale of one million opportunities, 3.397 defects result in a six-sigma process. The computation that gives the Sigma Quality Level statistic is
Sigma Quality Level = NormalQuantile(1 – defects/opportunities) + 1.5
Two of three quantities can be entered to determine the Sigma Quality Level statistic in the Sample Size and Power window:
•
Number of Defects
•
Number of Opportunities
•
Sigma Quality Level
When you click Continue, the sigma quality calculator computes the missing quantity.
Sigma Quality Level Example
As an example, use the Sample Size and Power feature to compute the Sigma Quality Level for 50 defects in 1,000,000 opportunities:
1.
Select DOE > Sample Size and Power.
2.
Click the Sigma Quality Level button.
3.
Enter 50 for the Number of Defects.
4.
Enter 1000000 as the Number of Opportunities. (See window to the left in Sigma Quality Level Example 1.)
5.
Click Continue.
The results are a Sigma Quality Level of 5.39. (See right window in Sigma Quality Level Example 1.)
Sigma Quality Level Example 1
Number of Defects Computation Example
If you want to know how many defects reduce the Sigma Quality Level to “six-sigma” for 1,000,000 opportunities:
1.
Select DOE > Sample Size and Power.
2.
Click the Sigma Quality Level button.
3.
Enter 6 as Sigma Quality Level.
4.
Enter 1000000 as the Number of Opportunities. (See left window in Sigma Quality Level Example 1.)
5.
Leave Number of Defects blank.
6.
Click Continue.
The computation shows that the Number of Defects cannot be more than approximately 3.4. (See right window in Sigma Quality Level Example 2.)
Sigma Quality Level Example 2