The chi square test is a nonparametrical statistical test used to determine whether there is a significant difference between observed frequencies and expected frequencies in one or more categories. The aim is to assess whether the difference between the observed and the expected frequencies is due to sampling variation or represents a real difference. Karl Pearson's chi square tests are used to assess three type of comparison: tests of goodness of fit, tests of independence, and tests of homogeneity. The difference between these three types of comparison lies in the situations for which each type should be used.