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Endgames Statistical Question

Treatment allocation in trials: cluster randomisation

BMJ 2014; 348 doi: (Published 17 April 2014) Cite this as: BMJ 2014;348:g2820
  1. Philip Sedgwick, reader in medical statistics and medical education
  1. 1Centre for Medical and Healthcare Education, St George’s, University of London, London, UK
  1. p.sedgwick{at}

Researchers investigated the effectiveness of neuromuscular training in reducing the rate of acute knee injuries in adolescent female football players. A cluster randomised controlled trial study design was used. The intervention was a 15 minute neuromuscular warm-up programme (targeting core stability, balance, and proper knee alignment) carried out twice a week throughout one season. A total of 230 football clubs were recruited from the female under 14 to under 18 series in eight regional districts of the Swedish Football Association, located in the middle and southern parts of Sweden. Clubs were randomised to the intervention or control using cluster allocation, stratified by regional district. In total, 121 clubs were allocated to the intervention group and 109 to the control group, and the clubs were followed for one season. The coaches of the clubs allocated to the control group were instructed to train and play as usual without any changes.1

The primary outcome was the rate of injury to the anterior cruciate ligament. Secondary outcomes were rates of severe knee injury (longer than four weeks’ absence from playing) and any acute knee injury. The authors reported that the neuromuscular warm-up programme significantly reduced the rate of anterior cruciate ligament injury in adolescent female football players. However, the absolute rate difference did not reach statistical significance, possibly because of the small number of events.

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

  • a) The football club was the unit of randomisation

  • b) The aim of cluster random allocation was to minimise contamination between football …

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