Statistical methods in epidemiology. III. The odds ratio as an approximation to the relative risk

PURPOSE: This paper introduces readers to case-control studies and how to analyse them. Specifically, the odds ratio statistic is discussed. The importance of the odds ratio in epidemiology is that it is used as an approximation to the true relative risk. METHOD: Data are presented in the form of 2 x 2 contingency tables, and a method for calculating the odds ratio is presented. An extension to 2 x k tables (where k > 2) is given, as is a graphical method for plotting odds ratios. Confidence intervals based on the Normal approximation are introduced. RESULTS: Some properties of the odds ratio statistic are illustrated by taking examples from the author's own teaching experiences. CONCLUSION: As long as the odds ratio is not used uncritically as an estimate of the relative risk, it remains an attractive statistic for epidemiologists to calculate.