why not percentage ?

Six Sigma – iSixSigma Forums Old Forums General why not percentage ?

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    If I am counting “defects” and it is say 1 per thousand, I can express it as 1000 DPMO or as 0.1 %. Does it really matter whether I express as percentage or as DPMO ? And I could have also expressed it as a sigma score – 4.6 sigma in this case. What is the additional benefit obtained by expressing it as a sigma score ? Why is it that in six sigma world, percentages are taboo and sigma scores are the way to go ?



    It is a good question.  I would say you can report process capability in any of the aforementioned forms, and there are numerous tables out there that practicioners use to do just that. Just be consistant in your methods. 
    I think the advantage to sigma/Z score (if there is one) is that it is used when dealing with continuous data, where you aren’t counting defects at all, you simply compare the allowed variability   (ie tolerance) to the actual variability (std dev) in your process to determine capability.  Hence, you simply calculate the mean and std dev and predict future capability from there.  And I would think you would need a much smaller sample size to get the same level of precision than if you used an attribute approach.
    But it will be interesting to hear the opinion of those more experienced.



    A major part of process improvement is communication which is why I tend to use percentages when talking to people outside of a core group of people as this is what people understand.
    On the question of using sigma score on continuous data, e.g. you can predict the outcome from the data etc once you have a score you can always translate it back into a percentage to allow people to undertsand it.


    Anton Javier

    Hi Zedd, you got a very valid point there!My take is that the choice of whether it shall be communicated as a
    percentage or as a sigma score largely depends on the intended
    audience. If your audience is your senior management, for
    example, and that their understanding of 6 sigma is just bare
    basics, you might wish to stick to your percentages presentation.
    You may, as you see fit, indicate a small take-away box at the end
    of the slide presentation to explain the % figures when translated
    into the 6 sigma world. What really matters is whether you:1 gained the support you need or that a new policy or guidance
    was created, reviewed, or repealed,
    2 were provided with new or additional resources
    3 attained the communication intent or purpose after the presentation. The issue of whether it was expressed as a sigma score or as % a
    percentage is another discussion.We communicate to express and not to impress anyway, right?Anton Javier

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