rank correlation coefficient
a numerical index reflecting the degree of relationship between two variables that have each been arranged in ascending or descending order of magnitude (i.e., ranked). It does not reflect the association between the actual values of the variables but rather that between their relative positions in the distribution. For example, placement in a marathon race could be correlated with the runners’ heights, but in this case the two variables—race outcome and height—would take the form first place, second place, and so on, and tallest, next tallest, and so on, respectively (rather than actual times run in the race and specific heights in feet and inches). Two of the most commonly used such indexes are the Spearman correlation coefficient and Kendall’s tau. Also called rank-order correlation coefficient.