Units of Analysis

Philip Sedgwick
Reader in medical statistics and medical education
Centre for Medical and Healthcare Education, St George’s, University of London, London, UK

December 12, 2014

Researchers assessed the effectiveness of a physical activity programme during one school year on the physical and psychological health of young schoolchildren. A cluster randomised controlled trial design was used. The physical activity programme consisted of three existing physical education lessons each week supplemented by a further two lessons, daily short activity breaks, and physical activity homework. Children in the control group received only the three existing physical education lessons each week.¹

In total, 28 classes at 15 elementary schools in Switzerland were recruited. Classes were randomly assigned to treatment in a 4:3 ratio; 16 classes at nine schools were assigned to the intervention and 12 classes at six schools were assigned to the control. There were 297 children allocated to intervention and 205 to control.

Primary outcome measures included for each child the change between baseline and follow-up in body fat, aerobic fitness, physical activity, and quality of life measures. The researchers reported that the school based multicomponent physical activity intervention improved physical activity and fitness and reduced adiposity in children.

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Competing interests: None declared.

1.    Kriemler S, Zahner L, Schindler C, Meyer U, Hartmann T, Hebestreit H, et al. Effect of school based physical activity programme (KISS) on fitness and adiposity in primary schoolchildren: cluster randomised controlled trial. BMJ 2010;340:c785.
2.    Sedgwick P. Cluster randomised controlled trials. BMJ 2012;345:e4654.
3.    Sedgwick P. The ecological fallacy. BMJ 2011;343:d4670.
4.    Hippisley­Cox J, Groom L, Kendrick D, Coupland C, Webber E, Savelyich B. Cross sectional survey of socioeconomic variations in severity and mechanism of childhood injuries in Trent 1992­7. BMJ 2002;324:1132.

Cite this as: BMJ 2013;346:f2128

This post is based on an article published in "Endgames" an educational series from The BMJ. Other Endgames articles are available here. Copyright BMJ Publishing Group 2013.