Intended for healthcare professionals

Endgames Statistical Question

Equivalence trials

BMJ 2013; 346 doi: (Published 11 January 2013) Cite this as: BMJ 2013;346:f184
  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}

Researchers evaluated the efficacy of 4% dimeticone lotion for the treatment of head louse infestation. A randomised controlled equivalence trial was performed. Control treatment was 0.5% phenothrin liquid, the standard treatment in the United Kingdom at the time of the trial. Treatments were applied twice, seven days apart, with dimeticone lotion for eight hours or overnight, and phenothrin liquid for 12 hours or overnight.1

The primary outcome was proportion of participants cured of infestation after the second application, regardless of whether reinfestation occurred later. The trial was designed to demonstrate therapeutic equivalence with an equivalence margin of 20%. Participants were young people (4-18 years) and adults with active head louse infestation. In total, 127 participants were allocated to the intervention (dimeticone lotion) and 125 to control (phenothrin liquid).

Analysis by intention to treat indicated that 89 of 127 (70%) participants treated with dimeticone were cured compared with 94 of 125 (75%) treated with phenothrin (difference −5%, 95% confidence interval −16% to 6%). Analysis by per protocol indicated that 84 of 121 (69%) participants were cured with dimeticone compared with 90 of 116 (78%) with phenothrin, a difference of −8% (−19% to 3%).

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

  • a) The control treatment of 0.5% phenothrin liquid is described as an active control

  • b) The null hypothesis for the equivalence trial stated no difference between treatments in cure rate in the population from where the participants were selected

  • c) Therapeutic equivalence in cure rate to within 20% was demonstrated between treatments


Answers a and c are true, whereas b is …

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