Specific agreement on ordinal and multiple nominal outcomes can be calculated for more than two raters

Objective: The concept of specific agreement (SA) has been proposed for dichotomous outcomes for two or more raters. We aim to extend this concept for variables with more than two ordinal or nominal categories and more than two raters.

Study design and setting: We used two data sets: four plastic surgeons classifying photographs after breast reconstruction on a 5-point ordinal scale and six raters classifying psychiatric patients into five diagnostic categories. For m raters, all (i.e., m(m-1)/2) pairwise agreement tables were summed to calculate the observed agreement (OA), SA and conditional probabilities. The 95% confidence intervals were obtained by bootstrapping.

Results: SA was calculated for each ordinal or nominal category to examine when one of the raters scored in a specific category, what is the probability that the other raters scored in that same category. And suppose one of the raters scored X_1, what is the probability that the other raters scored X₁ or any of the other categories (conditional probability). It appeared, for example, that among the psychiatric disorders, depression and personality disorders were often mixed up, whereas neurosis was rarely mixed up with schizophrenia.

Conclusion: The concept of SA for variables with ordinal and multiple nominal categories provides relevant clinical information. The extension to conditional probabilities of alternative categories broadens the clinical application with examining which categories are most often mixed up.