Intended for healthcare professionals

Endgames Statistical Question

Sample size and power

BMJ 2011; 343 doi: (Published 08 September 2011) Cite this as: BMJ 2011;343:d5579
  1. Philip Sedgwick, senior lecturer in medical statistics
  1. 1Centre for Medical and Healthcare Education, St George’s, University of London, Tooting, London, UK
  1. p.sedgwick{at}

Researchers carried out a randomised controlled trial to compare the effectiveness of cryotherapy with that of salicylic acid for treating plantar warts. Participants were eligible if aged 12 years or over. Those randomised to cryotherapy had liquid nitrogen delivered by a healthcare professional, with a maximum of four treatments, each 2-3 weeks apart. Participants randomised to 50% salicylic acid (Verrugon) treated themselves daily for a maximum of eight weeks. The trial was conducted as a superiority study.1

The primary outcome was complete clearance of all plantar warts at 12 weeks. The cure rate for salicylic acid was assumed to be 70% at 12 weeks, and the smallest effect of clinical interest was a difference in cure rates of 15%. To demonstrate the smallest effect of clinical interest with 80% power at the 5% critical level of significance, a sample size of 120 patients in each treatment group was needed, or 133 patients in each group after allowing for 10% attrition (266 patients in total).

The proportion of participants with complete clearance of all plantar warts at 12 weeks was …

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