Box Cox

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    Tom Joyce

    Can someone explain why data becomes Non Normal as it approaches Zero. Two why does Box Cox not transform negative numbers.


    Robert Butler

    The geometric mean is an integral part of the Box-Cox transform.  In order to compute the geometric mean you must take the log of all of your numbers and the log of a negative number is undefined.  I’m not sure what you mean by your question about non-normaility as numbers approach zero.  If you have a unilateral tolerance where zero represents perfection then as you drive your process closer and closer to that limit the only skew (error) that can occur is to one side and your data will, in general, become more and more non-normal the closer you approach zero as a limit.

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