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  • #49609

    Edwards
    Participant

    Hi All,
    Can I use Sigma level for non-normal data/ distribution?
    Your advice would be greatly appreciated. Thanks!
    David

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    #169746

    Craig
    Participant

    It would not be abnormal to use non-normal data for sigma calculation. A cpk can be calculated for non-normal data, so why can’t a sigma value? (Transform the data)

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    #169791

    Ropp
    Participant

    Hi hacl,
    The data cannot be transformed. The best fit distribution is log-normal. Yes, the Cpk can be calculated with log-normal distribution.
    I would like to understand on what is the potential error or assumption to be made to calculate sigma level for non-normal data, since the sigma level is derive from a normal distribution.
    Thanks!

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    #169793

    MS
    Participant

    I know that we are supposed to be software agnostic (Note: I do not work for Minitab – but use the software) – but Minitab offers non-normal capability analysis. Since you have identified the distribution used to model, you can select
    Stat > Quality Tools > Capability Analysis > Nonnormal
    then select the distribution that you wish to use as the model from the “fit data with” dropdown box.
    Then click on the options button and click the radio button for disply “Benchmark Z’s (sigma level). That will give you the sigma level for your process.
    If you do not have minitab, I imagine that other software packages offer this feature. Do a search in the help for “Non-normal capability analysis”
    Hope this helps!

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    #169794

    Brandon
    Participant

    Also note Sigma XL offers some very cost effective Excel based software that does essentially all that Mintab does. They are advertised on this site.
    PS: I don’t work for them, just aware of what they do.

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