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Comparing medians of unequal populations

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  • #39240

    Picklyk
    Participant

    I have 2 populations, before and after improvements.  the before is 100 samples, the after 400 samples.  The data is not normal.  I want to compare medians using Mann-Whitney and say they are “different”.  Is there a problem in comparing the medians of these unequal populations? 

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    #119003

    Ken Feldman
    Participant

    No.

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    #119004

    luke skywalker
    Participant

    You might check the 2 samples graphically to assess the shapes. The Mann-Whitney test loses power as the shapes of the distributions become different. If you get 2 similar histograms, use M-W. If not, try Mood’s Median.

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    #119005

    miranax
    Participant

    what kind of improvements are you looking for?  perhaps the medians have changed, but the “improvement” increased variation.  This will not be detected by the Mann-Whitney.

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    #119008

    Picklyk
    Participant

    Miranax, Luke and Darth
    Thanks.  The dist. are similaer in shape.  Both M-W and Moods say the medians are different.  Mira – I am looking to reduce median, which I did from 33.8 to 22.8.  Did Levenes which said the variances are different as well.  So everything looks like a true improvement. 

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    #119934

    vin
    Member

    Hii
    For comparing Comparing 2 Distributions, – ( when data is not normal ) you can go for the Test of Medians..
    i suggest to do Kuskall- Wallis test of Medians.. here…
    regards
    vin

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    #119938

    Whitehurst
    Participant

    Kruskal wallis or Moods Median test
     

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    #120180

    Cravens
    Participant

    If Levene’s shows that the standard deviations are different, what about the two-sample t without pooling, given your large sample sizes?  (See MINITAB help for: Stat > Nonparametrics > Mann-Whitney)

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