P values or confidence intervals?BMJ 2013; 346 doi: https://doi.org/10.1136/bmj.f3212 (Published 17 May 2013) Cite this as: BMJ 2013;346:f3212
- Philip Sedgwick, reader in medical statistics and medical education
- 1Centre for Medical and Healthcare Education, St George’s, University of London, Tooting, London, UK
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 …