Intended for healthcare professionals

Endgames Statistical Question

Sources of bias in randomised controlled trials II

BMJ 2010; 341 doi: (Published 22 December 2010) Cite this as: BMJ 2010;341:c7053
  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}

Previous questions described a cluster randomised controlled trial that assessed the long term effects of an obesity prevention programme in schools.1 2 The intervention was a focused education programme delivered over one school year that promoted a healthy diet and discouraged consumption of carbonated drinks. The control treatment was no intervention.3

The clusters were classes of children aged 7-11 years within schools. Classes were allocated to a treatment group by using a random sequence generated by a researcher not involved in recruitment. A second researcher who was unaware of the sequence recruited 29 classes from six schools. As classes were recruited they were allocated to the next treatment in the sequence. Fifteen classes totalling 325 children were randomised to the active intervention and 14 classes totalling 319 children to the control.

The main outcome measures were change in anthropometric measures such as height, weight, and waist circumference. Three years after baseline, outcome measures were obtained for 219 (67.4%) children …

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