Variances and Covariances of Kendall's Tau and Their Estimation

We generalize the formulas derived by Daniels and Kendall (1947) for the variance of the sample tau correlation. It is assumed that multivariate data are sampled from a population, and sample taus between pairs of variables are being used to estimate their population counterparts. Expressions for the variance of tau-a are generalized to allow for ties on either variable, and we further provide an expression for the covariance between two taus, including the special case where there is a variable in common. Unbiased estimators of the variance and the covariances are also derived for use in small samples. The variances and covariances of tau-a are used to provide asymptotic variances for tau-b and Somers' d.