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Proportions, Odds, and Risk¹

¹ From the Departments of Radiology (C.L.S.) and Biostatistics (C.W.G.), University of Florida College of Medicine, PO Box 100374, Gainesville, FL 32610. Received July 2, 2003; revision requested July 30; revision received August 4; accepted August 13. Address correspondence to C.L.S. (e-mail: sistrc@radiology.ufl.edu).

View larger version (33K): [in a new window]   Figure 1. Diagram shows possible patterns of observed data in a 2 x 2 table. Cells with black circles contain relatively large numbers of counts. With pattern A, n11 and n22 are large, suggesting that when X is present, Y is "yes." With pattern B, n12 and n21 are large, suggesting that when X is absent, Y is "yes." With pattern C, n11 and n21 are large, suggesting that Y is "yes" regardless of X. With pattern D, n12 and n22 are large, suggesting that Y is "no" regardless of X. With pattern E, n11, n12, n21, and n22 are all about the same, suggesting that Y is "yes" and "no" in equal proportion regardless of X.

View larger version (26K): [in a new window]   Figure 2. Graph shows relationship between OR and probability of outcome in unexposed group. Curves represent values of underlying RR as labeled. Rectangle formed by dotted lines represents suggested bounds on OR and probability of outcome in the unexposed group within which no correction from OR to RR is needed. When OR is more than 2.5 or less than 0.5 or probability of outcome in the unexposed group is more than 0.1, a correction (calculation in text) should be applied.