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 impo rtance of the odds ratio in epidemiology is that it is used as an approxima tion to the true relative risk. Method: Data are presented in the form of 2 x 2 contingency tables, and a m ethod for calculating the odds ratio is presented. An extension to 2 x k ta bles (where k > 2) is given, as is a graphical method for plotting odds rat ios. Confidence intervals based on the Normal approximation are introduced. Results: Some properties of the odds ratio statistic are illustrated by tak ing examples from the author's own teaching experiences. Conclusion : As long as the odds ratio is not used uncritically as an estim ate of the relative risk, it remains an attractive statistic for epidemiolo gists to calculate.