This document discusses Spearman's rank correlation coefficient (Spearman's rho), which is a nonparametric measure of statistical dependence between two variables. It explains that Spearman's rho measures the strength and direction of association between two ranked variables. The document provides an example of calculating Spearman's rho to explore the correlation between fertilizer amounts and crop yields for 5 different crops. It finds a very strong positive correlation (Spearman's rho = 0.9) between fertilizer and crop yield.