USE OF THE STANDARD ERROR AS A RELIABILITY INDEX OF INTEREST - AN APPLIED EXAMPLE USING ELBOW FLEXOR STRENGTH DATA

The intraclass correlation coefficient(ICC) and the standard error of measurement (SEM) are two reliability coefficients that are reported f requently. Both measures are related; however, they define distinctly different properties. The magnitude of the ICC defines a measure's abi lity to discriminate among subjects, and the SEM quantifies error in t he same units as the original measurement. Most of the statistical met hodology addressing reliability presented in the physical therapy lite rature (eg, point and interval estimations, sample size calculations) focuses on the ICC. Using actual elbow flexor make and break strength measurements, this article illustrates a method for estimating a confi dence interval for the SEM, shows how an a priori specification of con fidence interval width can be used to estimate sample size, and provid es several approaches for comparing error variances (and square root o f the error variance, or the SEM).