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nonparametric tests

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Features of nonparametric statistical methods include:

  • nonparametric statistical tests can be used when the data being analysed is not a normal distribution
  • many nonparametric methods do not use the raw data and instead use the rank order of data for analysis
  • nonparametric methods can be used with small samples
  • two commonly used nonparametric methods are:
    • the Mann-Whitney U test
      • this is the nonparametric equivalent of the unpaired t-test
      • applied when there are two independent samples
    • the Wilcoxon matched-pairs signed ranks test
      • equivalent to the paired t-test
    • both of these tests have a power-efficiency of 95.5% when compared to their parametric equivalents (this means that the equivalent parametric test would be just as effective with a sample size that is 4.5% smaller than used in the nonparametric tests). The power-efficiency illustrates the fact that if all conditions of normality (of the sample distribution) are met, then nonparametric methods waste data


  1. Doctor (March 22nd 2005):33-5.