Intended for healthcare professionals

Education And Debate

Expressing the magnitude of adverse effects in case-control studies: “the number of patients needed to be treated for one additional patient to be harmed”

BMJ 2000; 320 doi: (Published 19 February 2000) Cite this as: BMJ 2000;320:503
  1. Lise M Bjerre, doctoral candidatea,
  2. Jacques LeLorier, chief (
  1. Department of Epidemiology and Biostatistics, Faculty of Medicine, McGill University, Montreal, Canada H3A 1A2
  2. Pharmacoepidemiology and Pharmacoeconomy Research Unit, Centre de Recherche, Centre Hospitalier de l'Université de Montréal, Campus Hôtel-Dieu, Montreal, Canada H2W 1T8
  1. Correspondence to: J LeLorier
  • Accepted 11 August 1999

The need to express estimates of risk in an understandable manner is a challenge faced regularly by those who work with the results of epidemiological studies and try to convey their meaning to others. This is not an easy task, as is illustrated by the recent “pill scare” in the United Kingdom, in which there was much confusion over the clinical importance of the scientific information that was made public. Furthermore, practising clinicians also need a readily understandable tool for weighing the risks of various treatments. Ideally, this should be feasible without recourse to complicated statistical concepts. In this paper, we propose a simple and intuitively understandable method for expressing the results of case-control studies.

Summary points

Results of epidemiological studies need to be expressed in understandable terms if they are to be of practical use to clinicians and policy makers

Case-control studies are often used to study adverse effects of treatment; odds ratios from these are used to express the magnitude of adverse effects, but are not intuitively understandable estimates of risk

A more understandable and informative means of expressing the risk of adverse events in case-control studies is “the number of patients needed to be treated for one additional patient to be harmed”

This is calculated from the odds ratio and the unexposed event ratemdash;that is, the rate of occurrence of the adverse event of interest in people not exposed to the treatment

Evidence from randomised controlled trials

Any intervention or exposure may have desirable and undesirable effects. Desirable effects are usually the intended effects of a treatment. These will often (at least for pharmacological interventions) have been established in randomised controlled trials before an agent is released onto the market and introduced into clinical practice.

In the context of randomised trials on the desirable effects of treatments, Sackett et al proposed a method for …

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