The Following Data Are Ranks for Four Groups on a Test

Essay

The following data are ranks for four groups on a test. One requirement that is not shown in the data must be met before a Wilcoxon-Wilcox test of all possible comparisons can be made. State the requirement, and compare all four groups.
$$\begin{array} { r c r r } \mathrm { A } & \mathrm { B } & \mathrm { C } & \mathrm { D } \\\hline 3 & 8 & 5 & 10 \\9 & 11 & 6 & 15 \\1 & 4 & 13 & 20 \\7 & 2 & 18 & 12 \\14 & 17 & 21 & 22 \\16 & 19 & 24 & 23\end{array}$$