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Measurement Variability and Confidence Intervals in Medicine: Why Should Radiologists Care?¹

¹ From the Department of Radiology and Health Outcomes Policy and Economics (H.O.P.E.) Center, Children’s Hospital, 3100 SW 62nd Ave, Miami, FL 33155; and Departments of Orthopaedic Surgery and Biostatistics, Children’s Hospital, Harvard Medical School, Boston, Mass. Received September 17, 2001; revision requested November 12; revision received December 17; accepted January 21, 2002. Address correspondence to L.S.M. (e-mail: smedina@post.harvard.edu).

View larger version (20K): [in a new window]   Figure 1. Graph shows PSV of the common carotid artery in an Amazon Indian population. Note that as the sample size increases from 5 to 100 subjects, the SD decreases and the 95% CI becomes narrower.

View larger version (28K): [in a new window]   Figure 2. Graph summarizes the data from Figure 1. As the sample size increases, the sample mean and SD are a closer representation of the population’s mean and SD. In addition, as the sample size increases, the 95% CI narrows.