Cohen's kappa

Cohen's kappa coefficient (symbol κ, lowercase Greek kappa) is a statistic used to measure inter-rater reliability for qualitative or categorical data. It is generally thought to be a more robust measure than simple percent agreement calculation, as κ incorporates the possibility of the agreement occurring by chance. There is controversy surrounding Cohen's kappa due to the difficulty in interpreting indices of agreement. Some researchers have suggested that it is conceptually simpler to evaluate disagreement between items. Cohen's kappa coefficient ranges from -1 (complete disagreement) to 1 (complete agreement).