Kruskal-Wallis test :
The Kruskal-Wallis test is a non-parametric statistical test used to determine if there are significant differences among the median ranks of several groups. Unlike the parametric ANOVA test, which assumes a normal distribution of the data, the Kruskal-Wallis test does not assume any specific distribution of the data and can be used with ordinal, interval, or even some nominal data.
For example, suppose a researcher is interested in comparing the effects of three different types of exercise on weight loss. The researcher randomly assigns participants to one of the three exercise groups and measures their weight at the beginning and end of the study. The researcher can use the Kruskal-Wallis test to determine if there are significant differences in the amount of weight loss among the three exercise groups.
Another example is a study investigating the effects of different teaching methods on student test scores. The researcher randomly assigns students to one of four teaching methods and measures their test scores at the end of the semester. The Kruskal-Wallis test can be used to determine if there are significant differences in the median test scores among the four teaching methods.
To perform the Kruskal-Wallis test, the researcher first ranks the observations from each group from lowest to highest. The sum of the ranks for each group is then calculated and the Kruskal-Wallis statistic, H, is calculated using the following formula:
H = (n1n2…nr / N(N+1)) * Σ(Ri^2 / ni)
where n1, n2, …, nr are the number of observations in each group, N is the total number of observations, and Ri is the sum of the ranks for group i.
If the calculated value of H is greater than the critical value from a table of critical values, then the null hypothesis is rejected and there are significant differences among the median ranks of the groups.
One important consideration when using the Kruskal-Wallis test is the assumption of independence among the observations. If the observations are not independent, then the results of the test may be biased. For example, if the participants in the weight loss study are related or live in the same household, then their weight loss may not be independent of each other and the results of the Kruskal-Wallis test may be inaccurate.
Another limitation of the Kruskal-Wallis test is that it only allows for comparison of the median ranks among groups. If the researcher is interested in comparing the mean or other statistical measures, then a different test, such as the ANOVA, may be more appropriate.
Overall, the Kruskal-Wallis test is a useful non-parametric test for comparing the median ranks of several groups and can be used in a variety of research settings. However, it is important to carefully consider the assumptions of the test and whether it is the most appropriate test for the specific research question and data.