Non-parametric tests on the other hand are based on a statistical
model that has only very few assumptions.
None of these assumptions include making assumptions about the form
of the population distribution from which the sample was taken.
Whenever categorical or ordinal data are looked at, we should use non-parametric
Furthermore, if the data can be shown to be not normally distributed
non-parametric tests should be used.
Example of such test is Wilcoxon Mann-Whitney Test