Estimating rater agreement in 2x2 tables: Correction for chance and intraclass correlation

Many estimators of the measure of agreement between two dichotomous ratings of a person have been proposed. The results of Fleiss (1975) are extended, and it is shown that four estimators- Scott’s (1955) π coefficient, Cohen’s (1960) kˆ, Maxwell & Pilliner’s (1968) r₁₁, and Mak’s (1988) p˜-are interpretable both as chance-corrected measures of agreement and as intraclass correlation coefficients for different ANOVA models. Relationships among these estimators are established for finite samples. Under Kraemer’s (1979) model, it is shown that these estimators are equivalent in large samples, and that the equations for their large sample variances are equivalent. Index terms: index of agreement, interrater reliability, intraclass correlation, kappa statistic