AbstractThe kappa index is usually used for measuring the agreement between two observers when the scale is nominal. A modification of Cohen's kappa index was given by Krauth. The new estimator was biased and its large sample variance was obtained. An alternative estimator is developed here It is a ratio estimator and its mean square error is derived. A comparison with Cohen's estimator and Krauth's one is given by the examples used in the paper of Krauth.