Absolute and relative risksBMJ 2012; 345 doi: https://doi.org/10.1136/bmj.e5613 (Published 29 August 2012) Cite this as: BMJ 2012;345:e5613
- Philip Sedgwick, reader in medical statistics and medical education
- 1Centre for Medical and Healthcare Education, St George’s, University of London, Tooting, London, UK
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. …