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Kruskal-wallis One-way Analysis Of Variance




Since it is a non-parametric method, the Kruskal-Wallis test does not assume a Normal population, unlike the analogous one-way Analysis Of Variance .


METHOD

# Rank all data from all groups together.
# The test statistic is given by: K = (N-1) rac{\sum_{i=1}^g n_i(\bar{r}_{i\cdot} - \bar{r})^2}{\sum_{i=1}^g\sum_{j=1}^{n_i}(r_{ij} - \bar{r})^2}, where:
  • n_g is the number of observations in group g

  • r_{ij} is observation j from group i

  • N is the total number of observations across all groups

  • \bar{r}_{i\cdot} = rac{\sum_{j=1}^{n_i}{r_{ij}}}{n_i},

  • \bar{r} is the average of all the r_{ij}, equal to (N+1)/2.

  • :Notice that the denominator of the expression for K is exactly (N-1)N(N+1)/12.

  • # Finally, the P-value is approximated by \mathbf{P}(\chi^2_{N-g} \ge K). If some ni's are small the distribution of K can be quite different from this.



SEE ALSO




REFERENCES


  • William H. Kruskal and W. Allen Wallis. Use of ranks in one-criterion variance analysis. ''Journal of the American Statistical Association'' 47 (260): 583–621, December 1952.