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Examining intervention mechanisms of action using mediation analysis within a randomised trial of a whole-school health intervention
  1. Chris Bonell1,
  2. Elizabeth Allen2,
  3. Charles Opondo2,
  4. Emily Warren1,
  5. Diana Ruth Elbourne2,
  6. Joanna Sturgess2,
  7. Leonardo Bevilacqua3,
  8. Jennifer McGowan3,
  9. Anne Mathiot3,
  10. Russell M Viner3
  1. 1 Department of Public Health, Environments and Society, London School of Hygiene and Tropical Medicine, London, UK
  2. 2 Department of Medical Statistics, London School of Hygiene and Tropical Medicine, London, UK
  3. 3 Department of Social Science, UCL Institute of Child Health, London, UK
  1. Correspondence to Professor Chris Bonell, Department of Public Health, Environments and Society, London School of Hygiene and Tropical Medicine, London WC1H 9SH, UK; chris.bonell{at}


Background Interventions to modify school environments are effective in promoting young people’s health across outcomes, but mechanisms are poorly understood. We assessed mediation in a trial of the Learning Together intervention, building on the recent publication of results of effectiveness for reducing bullying and benefits across secondary outcomes and generally good implementation fidelity.

Methods Within a cluster-randomised trial involving 40 English schools, we examined student-reported and staff-reported school climate and student-reported involvement with delinquent peers at 24-month and 36-month follow-up, assessing the reliability of measures and whether these mediated health outcomes at a final follow-up.

Results Response rates and reliability were good for student-reported but not staff-reported measures. The intervention increased student-reported but not staff-reported-positive school climate but, like effects on student health outcomes, these manifested only at a final follow-up. The intervention reduced student-reported contact with delinquent peers at an interim follow-up. Student-reported potential mediators measured at the interim follow-up were associated with most health outcomes at the final follow-up. Adjustment for student-reported school climate and contact with delinquent peers at the interim follow-up did not reduce the associations between trial arm and our health outcomes.

Conclusion Despite being constrained by imperfect measures and by the late manifestation of impacts on student-reported school climate undermining ability to assess mediation, our study for the first time provides tentative evidence that mediation of intervention effects via improved climate and disengagement from delinquent peers is plausible. Our study provides the first evidence from a trial that whole-school interventions may work by modifying school environments and student relationships.

Trial registration number ISRCTN10751359.

  • adolescent
  • schools
  • environment
  • trial
  • behaviour

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  • Contributors CB and RVM: directed the trial from which the data are drawn. CB and EA: designed the analysis for this paper. CO: implemented this design and undertook the analysis. DRE: provided additional statistical expertise. CB: drafted the paper with inputs from EA, CO, DRE and RMV. EW, LB, JM, AM and JS: worked on the trial, contributed to the design of data collection instruments and contributed to the drafting of the paper.

  • Funding This paper presents independent research funded by the National Institute for Health Research (NIHR) in England under its Public Health Research Board (12/153/60) and the Education Endowment Foundation.

  • Disclaimer The views expressed in this publication are those of the authors and do not necessarily reflect those of the National Health Service (NHS), the NIHR or the Department of Health for England.

  • Competing interests RV is President of the Royal College of Paediatrics & Child Health.

  • Patient consent Obtained.

  • Ethics approval UCL (reference number 5248/001).

  • Provenance and peer review Not commissioned; externally peer reviewed.

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