On Similarity Coefficients for 2x2 Tables and Correction for Chance

This paper studies correction for chance in coefficients that are linear functions of the observed proportion of agreement. The paper unifies and extends various results on correction for chance in the literature. A specific class of coefficients is used to illustrate the results derived in this paper. Coefficients in this class, e.g. the simple matching coefficient and the Dice/Sørenson coefficient, become equivalent after correction for chance, irrespective of what expectation is used. The coefficients become either Cohen's kappa, Scott's pi, Mak's rho, Goodman and Kruskal's lambda, or Hamann's eta, depending on what expectation is considered appropriate. Both a multicategorical generalization and a multivariate generalization are discussed.