iSixSigma

Data Transformation

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  • #54102

    Manbir Singh
    Participant

    It is advised that if data is not normal then we do data transformation to make to Normal. Commonly used transformation is Box Cox. what is the need to transform data when we have non parametric tests available.

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    #193649

    Manbir Singh
    Participant

    Thanks Robert & Joel. This helps a lot.
    So can i say that even if the data is not normal i can go ahead with Anova and 1,2 Sample T tests as long as the data is too skewed and only few data points are outside control limits. However i need to figure out what distribution type it is to capture accurate process capability?

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    #193651

    Chris Seider
    Participant

    @talk2manbir

    If you are talking 5,10, 15, 20+% defective, don’t even bother coming up with the right distribution to get the most appropriate process capability. As @rbutler stated it well, go look for your causes and begin to eliminate the reasons. Customers don’t care what distribution the data follows, they care about the % out of specification.

    If you do a basic process capability (normal) using Minitab, just use the % or ppm observed in the lower left box. Make this your primary metric, and do what rbutler advised.

    Some advocate transformations, but I advise my belts to begin to solve the problems and then rationalization can occur as to which distribution might make sense.

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    #193681

    Darth
    Participant

    @joelatminitab Hey Joel, hope all is well. Circling back to the last comment the poster made, he concluded that he could go ahead and do ANOVA if the normality wasn’t too bad. Let’s not forget about the notion of equal variances. That might be a bigger issue than normality, eh?

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    #193683

    Joel Smith
    Participant

    @Darth – That’s right. Alternatively, the Welch Test is not sensitive to differences in variation. The Assistant Menu in Minitab uses the Welch Test for One-Way ANOVA while the Stat Menu uses the traditional F-Test.

    If the equal variances assumption is not met, I’d recommend Welch first before doing a nonparametric.

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    #193684

    Darth
    Participant

    @joelatminitab Just making sure you are still on top of things :-).

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    #193692

    Mike Carnell
    Participant

    @Darth @joelatminitab I am sure Joel feels much better knowing that you are checking up on him. It looks like you have picked up a Canadian accent?

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