Spearman's rank correlation coefficient (${\rho }_s=\mathrm{Corr}(\mathrm{rank}X,\mathrm{rank}Y)$)

A correlation coefficient computed from ranks, measuring monotone association. Ties need a stated ranking convention. A strong nonlinear but monotone relationship can have a strong Spearman rank correlation even when a straight-line summary in the original units is inappropriate.