Confidence Limits for Your ICC

Measuring and reporting an index of reliability has become a requirement for contributors to Physical Therapy.1 Currently, the reliability index most frequently cited in the physical therapy literature is the intraclass correlation coefficient (ICC), and guidelines have been established to help potential authors identify the type of ICC reported in their work.2,3 The application of these guidelines has aided readers in their interpretation of the ICC; however, I suggest that an area of deficiency continues to exist. Although it is a requisite practice to report the type of ICC, no standard exists for reporting the confidence one has in the magnitude of the ICC. For example, although the ICC was cited frequently in the most recent volume of Physical Therapy, the same statement cannot be made concerning the application of formal statistical tests supporting the magnitude of the reported coefficients. When statistical tests were applied, the reporting focused solely on the p value.… David E Krebs, PhD, PT, Associate Professor, Graduate Program in Physical Therapy, MGH Institute of Health Professions, 15 River St, Boston, MA 02108-3402