= Kendall rank correlation coefficient
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{title2=$\tau_a=(C-D)/\binom n2$}
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A family of ordinal association measures based on <concordant pairs> and <discordant pairs>. Without a tie adjustment, the coefficient is $\tau_a=(C-D)/\binom n2$: the <arithmetic mean>, over unordered pairs, of the product of the two coordinate comparison signs. Reversing one ordering reverses the sign, and strictly increasing relabellings preserve it. <Kendall's tau-b> makes an explicit adjustment for <tied pairs>. The name commemorates <Maurice Kendall>.
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