Relationship between Intraclass and Concordance Correlation Coefficients: Similarities and Differences

The intraclass and concordance correlation coefficients (ICC and CCC) are popular indices of reliability or agreement for continuous variables. While various versions of ICC are used according to study design, the CCC is solely used as an index of agreement among fixed raters. However, we considered other versions of CCC in connection with all types of ICC, as classified by McGraw and Wong (Psychological Methods 1: 30-46, 1996), and examined the similarities and differences between ICCs and CCCs. Because they were found to be compatibly similar to each other, it is considered that CCC has a wider range of applicability and ICC shares the framework of CCC including factors of precision and accuracy as well as applicability to pairwise sub-analysis among multiple raters. Although it is said that some version of ICC should selectively be used according to study purpose and design, useful information is added by the combined use with the other ICCs and CCCs and by pairwise sub-analysis. For a better understanding of the study results, additional information concerning the possible factors which could lower or enhance precision and/or accuracy will be necessary.