Iranian Journal of

There are situations in medical studies, wherein it is impossible to use the methods based on normal distribution (parametric methods). This paper objects to introduce common nonparametric methods and the inferences based on the methods in medical studies. Principles and method of calculations along with the software codes for common nonparametric methods and inference based on them were presented taking into account the considerations relevant to choose the nonparametric methods and their relative efficiency with examples in medical studies. In the situation where the assumptions are not satisfied, the nonparametric methods should be used without caution to lose the efficiency or even with higher efficiency of these methods. To compare a non-normal or ordinal variable between two groups Mann-Whitney test, to compare a non-normal or ordinal variable among more than two groups Kruskal-Wallis test, to compare a non-normal variable between two related situations or matched groups Wilcoxon test and to compare an ordinal variable between two related situations or matched groups Sign test should be used. In each of these tests the results of research based examples were presented along with the methods of their calculations. To assess the relation or difference in all types of medical studies, these tests are recommended considering the situation and purpose of study.