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Endgames Statistical Question

Randomised controlled trials: understanding confounding

BMJ 2015; 351 doi: (Published 25 September 2015) Cite this as: BMJ 2015;351:h5119
  1. Philip Sedgwick, reader in medical statistics and medical education1
  1. 1Institute for Medical and Biomedical Education, St George’s, University of London, London, UK
  1. Correspondence to: P Sedgwick p.sedgwick{at}

Researchers assessed the effectiveness of an exercise programme in reducing injurious falls among women at increased risk of falls and injuries. A multicentre parallel group randomised controlled trial study design was used. The intervention consisted of weekly supervised group sessions of progressive balance training offered in community based premises for two years, supplemented by individually prescribed home exercises. The control treatment was standard care. The study took place in 20 study centres throughout France. Participants were aged 75-85 years, living in their own home, and with reduced balance and gait capacities. In total, 706 women were recruited and randomly allocated to the intervention group (exercise programme; n=352) or control group (standard care; n=354). The random allocation of participants was stratified by study centre and body weight (<59 kg v ≥59 kg).1

The primary outcome was the rate of injurious falls (moderate and severe). Secondary outcomes included physical tests and perception of overall physical function. Of those women allocated to the intervention, 306 completed the trial protocol compared with 294 of those women allocated to control treatment. Analysis was by intention to treat. The risk of injurious falls was significantly reduced in the intervention group when compared with the control group (hazard ratio 0.81, 95% confidence interval 0.67 to 0.99). At two years, women in the intervention group performed significantly better on all physical tests and had significantly better perception of their overall physical function than women in the control group. It was concluded that the exercise programme was effective …

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