A primer on the intraclass correlation coefficient as a measure of reliability in medical research

The intraclass correlation coefficient (ICC) is a measure of reliability that is available in many forms, classified as one-way or two-way, random or mixed model, for single measures or as a mean of k measures, and as values for consistency or absolute agreement. Different forms of the ICC are appropriate in different clinical research contexts; each form has its own formula and interpretation. Literature on the ICC is sparse, and so concepts related to this statistic tend to be poorly understood, possibly resulting in a wrong form being used, and commonly resulting in poor reporting of the statistic in research papers. This article therefore explains the ICC and its forms in simple language, with examples where required. Practical guidance is provided for use and interpretation of the ICC. For most studies of test-retest reliability, or inter-rater reliability, the two-way mixed model ICC for single measures is appropriate, and the absolute agreement, along with its 95% confidence interval, should be reported.