Intended for healthcare professionals

Endgames Statistical Question

Absolute and relative risks

BMJ 2012; 345 doi: (Published 29 August 2012) Cite this as: BMJ 2012;345:e5613

This article has a correction. Please see:

  1. Philip Sedgwick, reader in medical statistics and medical education
  1. 1Centre for Medical and Healthcare Education, St George’s, University of London, Tooting, London, UK
  1. p.sedgwick{at}

Researchers evaluated the effectiveness of a single application of topical chloramphenicol ointment in preventing wound infection after minor dermatological surgery. A randomised placebo controlled, double blind, multicentre trial was performed. Placebo was a single application of paraffin ointment. A total of 972 minor surgery patients with high risk sutured wounds were recruited and randomised to a single topical dose of chloramphenicol ointment (n=488) or placebo (n=484). The primary outcome measure was incidence of infection on the agreed day of removal of sutures or sooner if the patient re-presented with a perceived infection.1

The researchers reported that the percentage of participants with an infection was significantly lower in the chloramphenicol group than in the placebo group (6.6% versus 11%; P=0.01). The relative risk of wound infection for chloramphenicol compared with placebo was 0.6 (95% confidence interval 0.39 to 0.91). The number needed to treat was 22.8.

Which of the following statements, if any, are true?

  • a) Each treatment group estimated the population at risk.

  • b) The intervention resulted in an absolute risk reduction of 0.044 compared with placebo.

  • c) The intervention resulted in a relative risk reduction of 0.4 compared with placebo. …

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