Coefficient Lambda for Interrater Agreement Among Multiple Raters: Correction for Category Prevalence

Fleiss's Kappa is an extension of Cohen's Kappa, developed to assess the degree of interrater agreement among multiple raters or methods classifying subjects using categorical scales. Like Cohen's Kappa, it adjusts the observed proportion of agreement to account for agreement expected by chance. However, over time, several paradoxes and interpretative challenges have been identified, largely stemming from the assumption of random chance agreement and the sensitivity of the coefficient to the number of raters. Interpreting Fleiss's Kappa can be particularly difficult due to its dependence on the distribution of categories and prevalence patterns. This paper argues that a portion of the observed agreement may be better explained by the interaction between category prevalence and inherent category characteristics, such as ambiguity, appeal, or social desirability, rather than by chance alone. By shifting away from the assumption of random rater assignment, the paper introduces a novel agreement coefficient that adjusts for the expected agreement by accounting for category prevalence, providing a more accurate measure of interrater reliability in the presence of imbalanced category distributions. It also examines the theoretical justification for this new measure, its interpretability, its standard error, and the robustness of its estimates in simulation and practical applications.