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Math Help - Test for normality - non-normal data ?

  1. #1
    Junior Member
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    Nov 2009
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    Question Test for normality - non-normal data ?

    My histograms shows the data are rightly skewed.
    So if the data are not normally distributed, i can't calculate the control limits or use the "tests for special causes", does it mean I have to convert non-normal data to normal ?
    if so, how do I convert
    So after conversion, how do I proceed from there? Will the data be "transformed" and I have to use the new figures?
    Do I need to do "sub-grouping" ? If so, how would you suggest I group the data so that I can do Variable Charts for Subgroups > Xbar-S (in minitab) ?

    Thanks !
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  2. #2
    MHF Contributor
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    Re: Test for normality - non-normal data ?

    Hey hazel.

    You might want to tell us what you are trying to do. Are you trying to do some standard test like estimate a mean (or do an ANOVA)? Maybe look at residuals?

    There are a couple of test-statistics that you can use to check normality: Things like Shapiro-Wilk and Kolmorogov-Smirnov help test for normality, but you should also try and find a function to create a "fitted" normal model of your data which should show a curve along your histogram.

    Combine these two things together will help you get an idea both in graphic and test-statistic and p-value form whether something may be normally distributed (Remember that things can come from some distribution but they won't necessarily reflect the entirety of that distribution: we usually have enough confidence if the sample size is large enough, but even this is not any gaurantee).
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