Intended for healthcare professionals

Endgames Statistical Question

Number needed to treat I

BMJ 2011; 342 doi: (Published 27 April 2011) Cite this as: BMJ 2011;342:d2463
  1. Philip Sedgwick, senior lecturer in medical statistics
  1. 1Section of 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.1 They conducted a placebo controlled double blind multicentre trial. Control treatment was a single application of paraffin ointment. In total, 448 minor surgery patients were allocated to chloramphenicol and 484 to placebo.

The primary outcome was incidence of infection within 30 days of the minor skin excision. The incidence of infection in the chloramphenicol group was 6.6% compared with 11.0% in the control group (P=0.010). The absolute reduction in infection rate was 4.4%, whereas the relative risk …

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