In statistics, the Kendall rank correlation coefficient, commonly referred to as Kendall's τ coefficient (after the Greek letter τ, tau), is a statistic used to measure the ordinal association between two measured quantities. A τ test is a non-parametric hypothesis test for statistical dependence based on the τ coefficient. It is a measure of rank correlation: the similarity of the orderings of the data when ranked by each of the quantities. It is named after Maurice Kendall, who developed it in 1938, though Gustav Fechner had proposed a similar measure in the context of time series in 1897. Intuitively, the Kendall correlation between two variables will be high when observations have a similar (or identical for a correlation of 1) rank (i.e. relative position label of the observations within the variable: 1st, 2nd, 3rd, etc.) between the two variables, and low when observations have a dissimilar (or fully different for a correlation of −1) rank between the two variables. Both Kendall's and Spearman's can be formulated as special cases of a more general correlation coefficient. Definition Let be a set of observations of the joint random variables X and Y, such that all the values of () and () are unique (ties are neglected for simplicity). Any pair of observations and , where , are said to be concordant if the sort order of and agrees: that is, if either both and holds or both and ; otherwise they are said to be discordant. The Kendall τ coefficient is defined as: Where is the binomial coefficient for the number of ways to choose two items from n items. Properties The denominator is the total number of pair combinations, so the coefficient must be in the range −1 ≤ τ ≤ 1. If the agreement between the two rankings is perfect (i.e., the two rankings are the same) the coefficient has value 1. If the disagreement between the two rankings is perfect (i.e., one ranking is the reverse of the other) the coefficient has value −1. If X and Y are independent and not constant, then the expectation of the coefficient is zero. An explicit expression for Kendall's rank coefficient is . Hypothesis test The Kendall rank coefficient is often used as a test statistic in a statistical hypothesis test to establish whether two variables may be regarded as statistically dependent. This test is non-parametric, as it does not rely on