For classical, norm-referenced test reliability, Cronbach's alpha has been shown to be equal to the mean of all possible split-half Pearson product-moment correlation coefficients, adjusted by the Spearman-Brown prophecy formula. For criterion-referenced test reliability, in an analogous vein, this paper provides the rationale behind, the analysis of, computational formulas for, and characteristics of a coefficient equal to the mean of all possible split-half coefficients of agreement. In addition, the relation of this coefficient to other test indices, including those of Harris and Livingston, is presented. (Author)