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Non Normal data

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  • #152779

    accrington
    Participant

    Can you explain what you want to do with the data?

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    #46304

    CK Hari
    Participant

    Hi,
     
    I would like to know the methods of dealing Non normal data.
     
    Thanks,
    Hari CK

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    #152816

    PV
    Participant

    Hi
    What sampling strategy did you use? If you had used rational subgrouping then chances are that you would have got normal data as (read central limit theorem). You probably took whatever data was available and did a normality test on that…
    Now if you do not have normal data then you measure of central tendency cannot be the mean and your measure of variation cannot be standard deviation (which measures to use would depend on the kind of distribution you have)
    If you want to do hypothesis testing with non normal data then you use non parametric tests (moods median, kruskall wallis etc)
    Basically do not fear that you have got non normal data but also do some intelligent sampling and data collection to begin with so that you have a better representation of data

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    #152823

    BB
    Participant

    There is no such thing as perfectly normal data.  You will never know precisely your data’s distribution.  Shewhart developed control charts without any assumption of data distribution.  Note that Shewhart did not rely on the Central Limit Theorem.
    Histograms also do not require any assumption of data distribution.

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    #152825

    BB
    Participant

    What???  No mention of Don Wheeler???  Are we converting you Steve???

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    #152826

    Ken Feldman
    Participant

    Interestingly enough I reread Shewhart’s book today for a paper I am writing.  He did mention a number of times that he was not concerned about nonnormality in the population because using rational subgrouping and subgroup averages, the issue was moot.  However, he does not ignore the concept of the CLT. 
    On page 289 Shewhart says, “We are now in a place to consider an additional and very important reason for choosing the average xbar of a sample to detect a change delta xbar and the standard deviation sigma to detect a change delta sigma.  The previous discussion has been limited to the assumption that the universe or distribution of standard quality is normal. 
    In Part IV, however, we saw that, no matter what the nature of the distribution function of the quality is, the distribution function of the the arithmetic mean approaches normality rapidly with increase in n, and in all cases the expected value of the means of samples of n is the same as the expected value Xbar of the universe.”
    This type of comment appears a few times in his book.  I think it is fair to say that the control chart works independent of the CLT and is robust to normality although Shewhart did not ignore the idea of the CLT in selecting the mean of samples as his format for identifying common cause and assignable cause.

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    #152830

    Chris Seider
    Participant

    I love non-normal data……If you have inputs recorded along with your non-normal output then you can identify some of the reasons (often referred to special causes) much easier….
    Do not be afraid….
    good luck

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