Kappa as a Parameter of a Symmetry Model for Rater Agreement

Abstract

If two raters assign targets to categories, the ratings can be arranged in a two-dimensional contingency table. A model for the frequencies in such a contingency table is presented for which Cohen’s kappa is a parameter. The model is based on two assumptions. First, the joint classification probabilities for the raters satisfy symmetry; second, the ratio of observed agreement to chance agreement is constant across categories. The model is illustrated using data from a study of the psychobiology of depression.