Abstract
The 36-Item Short Form Health Survey (SF-36) has been widely used to measure quality of life. Reliability has been traditionally assessed by intraclass correlation coefficient (ICC), which is equivalent to Cronbach's alpha theoretically. However, it is a scaled assessment of reliability and does not indicate the extent of differences because of measurement error. In this paper, total deviation index (TDI) is used to interpret the magnitude of measurement error for SF-36, and a new formula for computing TDI for average item score is proposed. The interpretation based on TDI is simple and intuitive by providing, with a high probability, the expected difference that is because of measurement error. We also show that a high value of ICC does not always correspond to a smaller magnitude of measurement error, which indicates that ICC can sometimes provide a false sense of high reliability. The methodology is illustrated with reported SF-36 data from the literature and from real data in the Arthritis Self-Management Program. Copyright © 2016 John Wiley & Sons, Ltd.
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