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===Mann-Whitney U test===
===Mann-Whitney U test===
The Mann-Whitney U test (also known as the Wilcoxon Rank Sum test) is commonly used for the comparison of two groups where the variable of interest is continuous but not normally distributed for at least one group. The basic concept behind the test is that all observations within each group are combined and then ranked in order of magnitude. These are then reassigned to their original groups, and the sums of the ranks in each groups are calculated and compared (by counting, for each observation in either of the groups, the number of observations in the alternate group which have a lower rank, and summing these together). Alternative methods are available for those cases where this approach would not be feasible.
==Comparing a quantitative variable between more than two groups==
==Comparing a quantitative variable between more than two groups==
===ANOVA (analysis of variance)===
===F-test===
The F-test is used when comparing the means of two or more quantitative variables, as is the case when conducting an ANOVA (analysis of variance). The basic concept of an ANOVA is that the total variation in a population can be viewed as being comprised of the variation between the groups (i.e. 'explained variation') and variation within the groups (i.e. 'unexplained variation'). The F statistic is based upon the ratio of a measure of the variance between groups (the 'mean square of treatment', MSTR) to a measure of the variance within groups (the 'mean square error', MSE).
===Kruskal-Wallis test===
===Kruskal-Wallis test===