BMJ 1999;318:1548-1551 ( 5 June )

Education and debate

Numbers needed to treat derived from meta-analysessometimes informative, usually misleading

Liam Smeeth, research fellow ^a,  Andy Haines, professor of primary care ^a,  Shah Ebrahim, professor of epidemiology and ageing ^b. 

^a Department of Primary Care and Population Sciences, Royal Free and University College Medical School, University College London, London NW3 2PF, ^b MRC Health Services Research Collaboration, Department of Social Medicine, University of Bristol, Bristol BS8 2PR

Correspondence to: Professor Ebrahim shah.ebrahim@bristol.ac.uk

The first 150 words of the full text of this article appear below.

The number needed to treatthe number of patients who must be treated to prevent one adverse outcomeis a widely used measure. ^{1 2} It is increasingly being calculated by pooling absolute risk differences in trials included in meta-analyses. ^{3 4} This option is available in statistical software and the Cochrane Database of Systematic Reviews. ^{5 6} In this paper, we examine pooled numbers needed to treat derived from trials and meta-analyses of interventions to prevent cardiovascular disease. We show that a pooled number needed to treat may be misleading because of variation in the event rates in trials, differences in the outcomes considered, effects of secular trends on disease risk, and differences in clinical setting. The number needed to treat should be derived by applying the relative risk reductions from treatment which have been estimated by trials or meta-analysis to relevant baseline risks for different types of patients. This provides a range of . . . [Full text of this article]

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