Tips for learners of evidence-based medicine: 1. Relative risk reduction, absolute risk reduction and number needed to treat.

Physicians, patients and policy-makers are influenced not only by the results of studies but also by how authors present the results. Depending on which measures of effect authors choose, the impact of an intervention may appear very large or quite small, even though the underlying data are the same. In this article we present 3 measures of effect — relative risk reduction, absolute risk reduction and number needed to treat — in a fashion designed to help clinicians understand and use them. We have organized the article as a series of “tips” or exercises. This means that you, the reader, will have to do some work in the course of reading this article (we are assuming that most readers are practitioners, as opposed to researchers and educators). The tips in this article are adapted from approaches developed by educators with experience in teaching evidencebased medicine skills to clinicians. A related article, intended for people who teach these concepts to clinicians, is available online at www.cmaj.ca/cgi/content/full/171/4/353/DC1.