Research methodology

How to interpret figures in reports of clinical trials

BMJ 2008; 336 doi: http://dx.doi.org/10.1136/bmj.39561.548924.94 (Published 22 May 2008)
Cite this as: BMJ 2008;336:1166

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  1. Stuart J Pocock, professor of medical statistics 1,
  2. Thomas G Travison, senior research scientist2,
  3. Lisa M Wruck, senior biostatistician3
  1. 1Medical Statistics Unit, London School of Hygiene and Tropical Medicine, London WC1E 7HT
  2. 2New England Research Institutes, Watertown, MA, USA
  3. 3Rho Inc, Chapel Hill, NC, USA
  1. Correspondence to: S J Pocock stuart.pocock{at}lshtm.ac.uk
  • Accepted 15 February 2008

A picture may be worth a thousand words but in medical research, caution Stuart Pocock, Thomas Travison, and Lisa Wruck, it is important to understand exactly what you are looking at

The graphical display of data is among the most powerful tools available for communicating medical research findings, given the increasing complexity of study designs and the mind’s preference for information conveyed in pictorial format.1 2 However, although general information is available on what constitutes an effective data display1 2 3 4 5 6 and what constitutes good practice in reporting trials,7 8 there is relatively little guidance on using figures to aid the presentation of trial results.9

Because figures are so effective in creating an enduring impression of results, their construction—and interpretation by readers—must be handled with care. We recently conducted a survey to determine the types of figures used most commonly in reports of clinical trials and to uncover the good, and not so good, practices that typically attend their use.10 Here, we highlight the important features of the most commonly used types of figures. In doing so, we hope to illustrate the hallmarks of figures that are likely to convey an impression consistent with valid trial conclusions and those aspects of figures that may, without careful interpretation, be misleading.

What comes up most

We examined all issues of five major general medical journals (Annals of Internal Medicine, BMJ, JAMA, Lancet, and New England Journal of Medicine) published from November 2006 to January 2007. The 77 reports of randomised trials included in these journal issues contained 175 figures (mean 2.3 figures per article). The four most common types of figure were flow diagrams (66 articles), Kaplan-Meier plots (32 articles), forest plots (21 articles), and repeated measures plots (20 articles) (table).10

View this table:

Figures in …

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