Spearman's rank correlation coefficient

Summary

Spearman's rank correlation coefficient $\rho_s$ is the Pearson correlation coefficient of the ranks of two variables. It measures how well the relation between the variables is described by a monotone function, not only by a straight line. It lies between $-1$ and $1$, and it equals $1$ exactly when one variable is a strictly increasing function of the other on the sample. It is unchanged by any strictly increasing change of scale of either variable.