Non-parametric Sample Size

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    Simon Davies

    We are performing a study to assess patient satisfaction with rehabilitation services. We plan to use a questionnaire with a Likert scale and the Mann-Whitney test to analyse the data.

    To calculate the required sample size we plan to first calculate the sample size for a 2 sample t-test, then use Asymptotic Relative Efficiency to adjust appropriately for Mann-Whitney.

    Could someone please advise us on how to go about selecting the appropriate delta in the t-test for this purpose? As the scale we are using has four discrete points, and the Mann-Whitney statistic is testing for median, does the expected difference between sample groups need to be greater than /1.0?


    Jeremy Miles

    This is pretty tricky, because the mann-whitney test is sensitive to the distribution. I would set up a simulation if I were you. It’s pretty straightforward.


    Dear Simon,
    as concerning your question about non-parametric sample size I think that is more useful for you using an ANOVA test. Good luck for your statistic analysis.
    Best regards, Giuseppina


    Dear Jeremy Miles,
    I think that is a good idea.
    Best regards, Giuseppina

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