Kendall's “Tau” Coefficient as an Index of Similarity in Comparisons of Plant or Animal Communities

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Abstract
Comparisons of ecologic communities are often limited to presentations of frequency lists in tabular or bar-graph form. Kendall's “Tau” coefficient is appropriate as a measure of rank correlation between such numerical profiles. A special problem arises where species in one community are absent from the other. Such species may either be assigned zero frequency and last-place ranking in the community where they are absent, or they may be dropped from both rankings, depending on which method yields the more conservative correlation. Tau has the same limits (± 1) as the familiar product-moment correlation coefficient. Moreover, as a nonparametric statistic it is free of considerations of distribution. Comparisons between soil-fauna communities from the Quetico-Superior forest area are used to illustrate the method. The problems of tied rankings, variance and significance of Tau, confidence limits for Tau, and the significance of a difference between Tau coefficients, are also considered.

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