A Guideline of Selecting and Reporting Intraclass Correlation Coefficients for Reliability Research

Abstract

Objective: Intraclass correlation coefficient (ICC) is a widely used reliability index in test-retest, intrarater, and interrater reliability analyses. This article introduces the basic concept of ICC in the content of reliability analysis.

Discussion for researchers: There are 10 forms of ICCs. Because each form involves distinct assumptions in their calculation and will lead to different interpretations, researchers should explicitly specify the ICC form they used in their calculation. A thorough review of the research design is needed in selecting the appropriate form of ICC to evaluate reliability. The best practice of reporting ICC should include software information, "model," "type," and "definition" selections.

Discussion for readers: When coming across an article that includes ICC, readers should first check whether information about the ICC form has been reported and if an appropriate ICC form was used. Based on the 95% confident interval of the ICC estimate, values less than 0.5, between 0.5 and 0.75, between 0.75 and 0.9, and greater than 0.90 are indicative of poor, moderate, good, and excellent reliability, respectively.

Conclusion: This article provides a practical guideline for clinical researchers to choose the correct form of ICC and suggests the best practice of reporting ICC parameters in scientific publications. This article also gives readers an appreciation for what to look for when coming across ICC while reading an article.

Keywords: Reliability and validity; Research; Statistics.

Figures

Fig 1

A flowchart showing the selection process of the ICC form based on the experimental design of a reliability study. The process involves the selection of the appropriate model (ie, 1-way random effects, 2-way random effects, or 2-way fixed effects), type (ie, single rater/measurement or the mean of k raters/measurements), and definition of relationship considered to be important (ie, consistency or absolute agreement).

Fig 2

Hypothetical data illustrating how different forms of ICC can give different results when applied to the same set of data and how the nature of the data affects the ICC estimates of different forms.

Fig 3

A flowchart showing readers how to interpret ICC in published studies. Values less than 0.5 are indicative of poor reliability, values between 0.5 and 0.75 indicate moderate reliability, values between 0.75 and 0.9 indicate good reliability, and values greater than 0.90 indicate excellent reliability.