Multi-Groups Design with Independent Samples: The Kruskal-Wallis One-Way Analysis of Variance by Ranks
This method functions like the conventional one-way analysis of variance. The null hypothesis is tested to determine if the differences among samples show true population differences or whether they represent chance variations to be expected among several samples from the same population. The test is based on the assumptions that ranks within each sample constitute a random sample for the set of numbers 1 through N (15) and that the variable being tested has a continuous distribution (4). Scores in all samples are combined and arranged in order of magnitude so that ranks can be given to each score. The lowest score is assigned the rank of 1. The scores then are replaced in their respective samples with appropriate ranks. The ranks for each sample are summed. The assumption is that mean rank sums (R) are equal for all samples and equal to the mean of the N ranks, (N + 1)12, if the samples (K) are from the same population (16). Both equal- and unequalsized samples can be used in this test because the sums of sample ranks (>R) are pooled in the equation. The statistic H used in this test can be defined by the equation: