Pitfalls and Important Issues in Testing Reliability Using Intraclass Correlation Coeffi cients in Orthopaedic Research

Abstract

Background: Intra-class correlation coeffi cients (ICCs) provide a statistical means of testing the reliability. However, their interpretationis not well documented in the orthopedic fi eld. The purpose of this study was to investigate the use of ICCs in the orthopedicliterature and to demonstrate pitfalls regarding their use. Methods: First, orthopedic articles that used ICCs were retrieved from the Pubmed database, and journal demography, ICC modelsand concurrent statistics used were evaluated. Second, reliability test was performed on three common physical examinationsin cerebral palsy, namely, the Thomas test, the Staheli test, and popliteal angle measurement. Thirty patients were assessed bythree orthopedic surgeons to explore the statistical methods testing reliability. Third, the factors affecting the ICC values were examinedby simulating the data sets based on the physical examination data where the ranges, slopes, and interobserver variabilitywere modifi ed. Results: Of the 92 orthopedic articles identifi ed, 58 articles (63%) did not clarify the ICC model used, and only 5 articles (5%)described all models, types, and measures. In reliability testing, although the popliteal angle showed a larger mean absolute differencethan the Thomas test and the Staheli test, the ICC of popliteal angle was higher, which was believed to be contrary to thecontext of measurement. In addition, the ICC values were affected by the model, type, and measures used. In simulated data sets,the ICC showed higher values when the range of data sets were larger, the slopes of the data sets were parallel, and the interobservervariability was smaller. Conclusions: Care should be taken when interpreting the absolute ICC values, i.e., a higher ICC does not necessarily mean lessvariability because the ICC values can also be affected by various factors. The authors recommend that researchers clarify ICCmodels used and ICC values are interpreted in the context of measurement.