What is a Six Sigma Level?
3.4 Defects per Million Opportunities in your process.
In the above example the defect rate is 50/1000. 6 Sigma quality is 3.4 / 1,000,000.
Where Does the 6 and Sigma come from?
Sigma is the easy one. It refers to the mathematical concept of standard deviation.
The 6 refers to the ability to fit the good outputs of your process to fit within 6 standard deviations (or sigmas, if you will). Another way to say it is that only 3.4 defects occur per every million opportunities fall outside your control limits.
This is easier to visualize.
If your process is normally distributed, it would look like this. All of the acceptable output of your process would fit between the outermost orange lines. By definition, all of your defective outputs would be represented by the area under the blue curve and outside those outermost orange lines.