Six Sigma is a methodology for continuous process improvement through a library of tools. But what’s the word Six Sigma means?

If you’re producing something or providing a service, you have a target for your service plus a lower and a higher limit to respect. If you don’t respect this limit, you have a defect.

Example: you are a process and estimate that the target process duration is six months. Then you say that you will respect this six-month plus or minus (lower and higher specification limit) of one month. So it's ok if you use the resources for this process from five to seven months. If you use it for eight months or four, you have a defect.

The sigma score says the quality of service, speaking of how many times you have a defect. Higher is the Sigma Level lower are the number of defects per million. The sigma level is the standard deviations between the real process mean and the closest specification limits. In image1 you can look at the SixSigma Score formula.

Image1 - Sigma score formula
Image1 – Sigma score formula
Example: In the image2, you have a visual example of a 2 Sigma process, where
- The target of the process is:12 +/- 6, so the SL, in this case, is 6;
- The mean (x-bar) of the real process is 10;
- The standard deviation (sigma) of the real process is 2.
If you apply the formula, you have a process of 2 sigmas. You can visually look that from 10 to 6; you have a difference of 4 (and one sigma is 2 in this case).
Image2 - 2Sigma process
Image2 – 2Sigma process

But what 2Sigma mean in terms of defect per million? You can use the table1:

Sigma Score% DefectsDPMO
Table1: from Sigma Score to defect and DPMO

In this case, we can say that 2Sigma is 30,09% of defects that means that for each milion, you have 308.538 defects. What we want is to achieve the better sigma level possible.

Remember that in this lecture, we talk about Specification Limit; they come from the customer’s specification. The control limit is something that we study next in this guide.

For the exam you need to remember that:

  • The math formula for the Sigma score;
  • Sigma Score is the number of Standard Deviation from the means of the process and the nearest limits;
  • Higher Sigma Score means lower defects.
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