Costner has argued that measures of association should be interpretable in terms of “proportional reduction in error” of prediction and described such an interpretation for one measure of ordinal association, Goodman and Kruskal’s gamma, but was unable to do so for Kendall’s tau-b.1 In this paper, a proportional-reduction-in-error interpretation for tan is presented, and certain limitations of such interpretations for ordinal variables are indicated. © 1969, Oxford University Press. All rights reserved.