title: Emergent approaches to the meta-analysis of multiple heterogeneous complex interventions.
abstract: BACKGROUND
  Multiple interventions meta-analysis has been recommended in the methodological literature as a tool for evidence synthesis when a heterogeneous set of interventions is included in the same review—and, more recently, when a heterogeneous set of complex interventions is included. However, there is little guidance on the use of this method with complex interventions. This article suggests two approaches to model complexity and heterogeneity through this method.
  
  
  CONCLUSIONS
  'Clinically meaningful units' groups interventions by modality or similar theory of change, whereas 'components and dismantling' separates out interventions into combinations of components and either groups interventions by the combination of components they demonstrate or extracts effects for each identified component and, possibly, interactions between components. Future work in systematic review methodology should aim to understand how to develop taxonomies of components or theories of change that are internally relevant to the studies in these multiple interventions meta-analyses.
  
  
  CONCLUSIONS
  Despite little meaningful prior guidance to its use in this context, multiple interventions meta-analysis has the potential to be a useful tool for synthesising heterogeneous sets of complex interventions. Researchers should choose an approach in accordance with their specific aims in their systematic review.
authors.affiliation: Centre for Evidence-Based Intervention, Department of Social Policy and Intervention, University of Oxford, Oxford, UK. gerardo.melendez-torres@spi.ox.ac.uk.
authors.forename: G J
authors.initials: GJ
authors.lastname: Melendez-Torres
authors.affiliation: Social Science Research Unit, UCL Institute of Education, University College London, University of London, London, UK. c.bonell@ioe.ac.uk.
authors.forename: Chris
authors.initials: C
authors.lastname: Bonell
authors.affiliation: Social Science Research Unit, UCL Institute of Education, University College London, University of London, London, UK. j.thomas@ioe.ac.uk.
authors.forename: James
authors.initials: J
authors.lastname: Thomas
mesh_terms: D035843:Biomedical Research / Q000379:methods / Q000706:statistics & numerical data*; D001699:Biometry / Q000379:methods; D003627:Data Interpretation, Statistical; D017408:Guidelines as Topic; D006801:Humans; D015201:Meta-Analysis as Topic; D012107:Research Design / Q000706:statistics & numerical data; D012196:Review Literature as Topic