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1 sided vs 2 sided hypothesis testing

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  • #49877

    Whitehurst
    Participant

    Hello,  Can anyone tell me if there are any advantags to using a one sided test as opposed to a two sided hypothesis test please?
    Cheers
    Joe

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    #171267

    Six Sigma guy
    Member

    it depends on what you want to do.. usually you would perform one tailed if you would like to determine a difference/significance in a particular direction( higher or lower) two tailed is performed if you like to determine if at all any difference/significance is present irrespective of the direction..generally said its better to do a two tailed test followed by a one tailed test..
    Cheers

    0
    #171268

    Whitehurst
    Participant

    That is part of what I am trying to understand.  If I first do a two sided test and the result is significant, showing there is a difference. Then based on the results, I do a one sided test, do both one and two sided results always come out significant or not significant providing the correct Ha direction has been set for the one sided tested?
    Hope this makes sense!
    Joe

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    #171282

    Erik L
    Participant

    Joe,
    It will always be easier to reject the null, and assume the alternative state with a directional alternative vs. a not equal alternative.  The best way to think about it would be to take the significance level you are interested in and then partition it against a reference null distribution.  Assuming 5% significance, and a one sided alternative, all 5 % is cumulatively allocated from the lower tail (assuming a l.t. alternative).  If it was a not equal alternative, you would equally partition 2.5% to either tail to make the delineation between signal and noise…The conservative approach is to default to the two-tailed alternative (mitigating alpha inflation-especially with lower sample sizes), unless there is a true reason that you are interested in a directional alternative.  Hope that helps with your question.
    Regards,
    Erik   

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    #171292

    Whitehurst
    Participant

    Thanks for this Erik

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