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Thread: can non parametric tests be modified for one tailed tests?

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    can non parametric tests be modified for one tailed tests?




    Hello all,
    super newbie here.
    just a quick question:
    I have been doing one tailed independent two sample t tests but now on other data I no longer have normal distributions so I wanted to use Mann Whitney U test, but I am not sure if these can be one tailed.
    I am using R so is it legitimate to specify the "alternative" argument as greater and if so (Im assuming so since it is an available option in R) do I then divide p/2 as I would for the t test to get a one tailed p value?
    any advice is appreciated.
    cheers
    J

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    Re: can non parametric tests be modified for one tailed tests?

    I have been doing one tailed independent two sample t tests
    Normally, you should not perform one-tailed tests.
    You make yourself half blind.
    but now on other data I no longer have normal distributions so I wanted to use Mann Whitney U test,
    If your total sample size is > 50 or so, non-normal
    distributions shouldn't matter much. One has to be
    more concerened with unequal variances (if samples
    have unequal size).
    do I then divide p/2 as I would for the t test to get a one tailed p value?
    Seems so. But normally you shouldn't do so.

    With kind regards

    K.

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    Re: can non parametric tests be modified for one tailed tests?

    Thanks for that.

    My samples are fairly small. I'm testing the direction hence the one tail.

    Cheers
    J

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    Re: can non parametric tests be modified for one tailed tests?


    I'm testing the direction hence the one tail.
    Many people test the direction, but you hardly ever
    see one-tailed tests. Cleary, the usual two-tailed
    test is absolutely ok for testing directional working
    hypotheses. Just having in mind what the direction
    of an effect might be does not justify one-tailed testing,
    because, as mentioned before, it makes the researcher
    half blind without good reason.

    With kind regards

    K.

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