title: Systematic review and meta-analysis of effects of community-delivered positive youth development interventions on violence outcomes.
abstract: BACKGROUND
  We systematically reviewed and meta-analysed evaluations testing the effectiveness of positive youth development (PYD) interventions for reducing violence in young people.
  
  
  METHODS
  Two reviewers working independently screened records, assessed full-text studies for inclusion and extracted data. Outcomes were transformed to Cohen's d. Quality assessment of included evaluations was undertaken using the Cochrane risk of bias tool. Effect sizes were combined using multilevel meta-analysis. We searched 21 databases, including MEDLINE, PsycINFO, CINAHL and CENTRAL, and hand-searched key journals and websites. We included studies where the majority of participants were aged 11-18 years and where interventions were delivered in community (not clinical or judicial) settings outside of normal school hours. We excluded studies targeting predefined physical and mental health conditions or parents/carers alongside young people. We defined violence as perpetration or victimisation of physical violence including violent crime.
  
  
  RESULTS
  Three randomised trials were included in this systematic review. Included evaluations each had design flaws. Meta-analyses suggested that PYD interventions did not have a statistically significant effect on violence outcomes across all time points (d=0.021, 95% CI -0.050 to 0.093), though interventions did have a statistically significant short-term effect (d=0.076, 95% CI 0.013 to 0.140).
  
  
  CONCLUSIONS
  Our meta-analyses do not offer evidence of PYD interventions in general having effects of public health significance in reducing violence among young people. Evaluations did not consistently report theories of change or implementation fidelity, so it is unclear if our meta-analyses provide evidence that the PYD theory of change is ineffective in reducing violence among young people.
authors.affiliation: Warwick Evidence, Division of Health Sciences, Warwick Medical School, University of Warwick, Coventry, UK.
authors.forename: G J
authors.initials: GJ
authors.lastname: Melendez-Torres
authors.affiliation: Social Science Research Unit, Evidence for Policy and Practice Information and Co-ordinating Centre (EPPI-Centre), UCL Institute of Education, London, UK.
authors.forename: Kelly
authors.initials: K
authors.lastname: Dickson
authors.affiliation: DECIPHer, School of Social Sciences, Cardiff University, Cardiff, UK.
authors.forename: Adam
authors.initials: A
authors.lastname: Fletcher
authors.affiliation: Social Science Research Unit, Evidence for Policy and Practice Information and Co-ordinating Centre (EPPI-Centre), UCL Institute of Education, London, UK.
authors.forename: James
authors.initials: J
authors.lastname: Thomas
authors.affiliation: Social Science Research Unit, Evidence for Policy and Practice Information and Co-ordinating Centre (EPPI-Centre), UCL Institute of Education, London, UK.
authors.forename: Kate
authors.initials: K
authors.lastname: Hinds
authors.affiliation: DECIPHer, School of Social and Community Medicine, University of Bristol, Bristol, UK.
authors.forename: Rona
authors.initials: R
authors.lastname: Campbell
authors.affiliation: DECIPHer, School of Social Sciences, Cardiff University, Cardiff, UK.
authors.forename: Simon
authors.initials: S
authors.lastname: Murphy
authors.affiliation: Department of Social and Environmental Health Research, London School of Hygiene and Tropical Medicine, London, UK.
authors.forename: Chris
authors.initials: C
authors.lastname: Bonell
keywords: CHILD HEALTH; SOCIAL SCIENCE; VIOLENCE