Published Studies of Interrater Reliability Often Overestimate Reliability: Computing the Correct Coefficient

It is erroneous to generalize the interrater reliability coefficient estimated from two or more raters rating only a (small) portion of the sample to the rest of the sample data for which only one rater is used for scoring, although such generalization is often made implicitly in practice. If the interrater reliability estimate from part of a sample is available, the score reliability for the rest of the sample data for which only one rater is used for scoring can be estimated both within the framework of classical reliability theory and that of generalizability theory. As intuitively expected, score reliability when only one rater is used for scoring is lower than the score reliability for which two raters are used. The authors provide a sample of published studies in different disciplines that inappropriately generalized reliability coefficients involving several raters to scores generated by a single rater.