Intended for healthcare professionals

Endgames Statistical Question

P values or confidence intervals?

BMJ 2013; 346 doi: (Published 17 May 2013) Cite this as: BMJ 2013;346:f3212
  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}

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

The critical level of significance for statistical testing was set at 0.05 (5%). The proportion of participants with an infection was significantly lower in the chloramphenicol group than in the placebo group (6.6% v 11.0%; difference −4.4%, 95% confidence interval −7.9% to −0.8%; P=0.010). The researchers concluded that the application of a single dose of topical chloramphenicol to high risk sutured wounds after minor surgery produced a statistically significant yet moderate absolute reduction in infection rate.

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

  • a) The P value provides a direct statement about the size of the difference between groups in …

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