Abstract
T topic of this article in the calculation skills series is the calculation of epidemiological measures from clinical trial data. The article will not discuss the relative merits of different kinds of clinical trial; instead, an excellent series of tools supporting the critical appraisal of medical literature is available from the Centre for Evidence Based Medicine (CEBM, 2011) The clinical trials selected to illustrate each calculation were chosen at random and have not been critically appraised by the author. We are simply going to look at how to calculate some of the measures, and what they mean in practice. The measures we will look at are: relative risk, relative risk reduction and absolute risk reduction. Relative risk is the ratio of the probability of an event occurring in an exposed group compared with the probability of it occurring in a non-exposed group. It can be used to analyse clinical trial data both for benefits and risks of treatment or to describe the risk of an event occurring. If the relative risk is 1, this means there is no difference in risk between the experimental and control groups. Relative risk reduction is the percentage reduction in risk between the experimental and control groups. Absolute risk reduction is the difference between the event rate in an experimental group receiving a particular treatment compared with the event rate in the control group. The control group could receive either placebo or a different drug. The equation to calculate relative risk is:
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