Clinical research is increasingly based on the empirical studies and the results of these are usually presented and analyzed with statistical methods. Therefore discuss frequently used statistical tests for different type of data set under assumption of normality or non-normality. The statistical tests applied when normality (and homogeneity of variance) assumptions are satisfied otherwise the equivalent non-parametric statistical test used. Advice will be presented for selecting statistical tests on the basis of very simple cases. It is therefore an advantage for any physician or researcher he/she is familiar with the frequently used statistical tests, as this is the only way he or she can evaluate the statistical methods in scientific publications and thus correctly interpret their findings.