What is the Meaning of Six Sigma?

What is the meaning of Six Sigma
What is the meaning of Six Sigma

What is the meaning of Six Sigma

Six Sigma starts with the view that waste is defective products, to remove waste you find the root cause of defects (using statistical or process analysis methods) then design an improvement or replace a fault so that the defects no longer occur.

Using a manufacturing example, let’s say you have a factory that produces widgets.  The widgets are all supposed to be a certain size, shape, and weight.  You sample your output and discover that out of every 1,000 widgets fifty are flawed (e.g. wrong size, shape, or weight).  To correct this you could use Six-Sigma to determine what’s causing the variation/defects and work to reduce them to a six-sigma level.

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.

A six sigma capable process will have Cp = 2.0, Cpk = 1.5 and DPMO = 3.4

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.

What is the Meaning of Six Sigma?

What is the Meaning of Six Sigma?

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.


16 comments… add one
  • Dr.K.Kumar

    How do I take six sigma exam from Villanova university?can you comment on Moresteam and Villanova university exam.can we take the the exam in parts or at the sam grime.thankig you.Regrds.Dr.K.Kumar

    • Hi Dr. Kumar,

      You can take the Villanova exam by registering for their Six Sigma Black Belt course. The exam comes after 6-8 weeks of training (if memory serves) and after you complete the example project.

      I ma not personally familiar with Moresteam but some colleagues have recently used their training to prepare – and pass – the ASQ Six Sigma Black Belt exam.

      Hope that helps!

      • mohanad_mahdy@hotmail.com


        i am interested in pursuing lean six sigma black belt. After some research I ended up with ASQ , LSSA (APMG), IASSC.

        appreciate if you could guide me on selecting one of them. I am struggling to select one of them or understand the differences between them .


  • guest

    There is an error in the diagram of six sigmas, it only shows three sigmas. (99.5%) Six sigmas is six standard deviations on a single side of the mean.

    • No, that is incorrect. The diagram is accurate.

      • mewen

        I was under the same impression, Six Sigmas on either side, otherwise you end up with 5 bad parts in 1000, not the 3.4 DPMO. I have a hard time finding this information elsewhere on the internet.

        Can you elaborate on why it is 3 sigmas on either side and not 6?

        • Hi Michael,

          I’ll look for a concise way to demonstrate this. In the meantime, please refer to these articles (and videos and images therein.)

          Z Scores and Process Sigma.

          How to determine baseline sigma.

        • Short answer is that the center of curve is the mean of the process. 6 sigmas represent nearly all of the possible outcomes. If the center is the mean, you’d expect half of the possibilities to fall on either side (in a normal distribution.) Half of 6 is 3, thus 3 sigmas on each side of the center.

          • jesch

            That’s not true…A six sigma process operates with Cp=2
            So the natural tolerance needs to be 6 sigma on either side of the mean.

  • Stevensixsigma

    Hi, I am new to Six Sigma but have lots of experience in working on lean projects. How do i start with six sigma . Is yellow belt certification from ASQ is the right way to start. Is there any study materials you suggest.

    • Hi Steven,

      I’ve really seen no use in pursuing yellow belts. Perhaps it’s a good way to quickly get recognized for knowledge you already have and add a resume item, but even then I’ve not seen employers value them. Perhaps other people’s experiences are different.

      I think the right way to address your question is to begin with the end in mind. What is it that you want to accomplish. If you want to be able to effectively participate in lean six sigma projects and begin to get an understanding of the whole, I’d pursue the green belt. If you are looking to be a change agent and lead six sigma projects, I’d pursue Black Belt training and certification.

      Either way, I think this website has the best available (and free!) study guides. Start by answering the survey here – it will help me provide exactly what you are looking for and will sign you up for our free email series that will logically lay out a course of action.

      Remember, you can always see what the ASQ Green Belt exam or the ASQ Black Belt exam entails by clicking those links.

  • Corwin Evans

    I would like to obtain study material for Black Belt training and certification

  • ahmadrafiq90

    There is an error in the last diagram. 6 sigma means +/- 6 sigma which means 6 deviations on either side.

  • Monica N

    I have to agree with the other posters and say that the graph is incorrect. Please do us a favor and correct this page, as I’m afraid it will be misleading to people who are using it as a study guide. See below website for a correct illustration of a six sigma graph:


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