title: Systematic reviews, overviews of reviews and comparative effectiveness reviews: a discussion of approaches to knowledge synthesis.
abstract: BACKGROUND
  The Cochrane Collaboration has been at the forefront of developing methods for knowledge synthesis internationally.
  
  
  OBJECTIVE
  We discuss three approaches to synthesize evidence for healthcare interventions: systematic reviews (SRs), overviews of reviews and comparative effectiveness reviews.
  
  
  METHODS
  We illustrate these approaches with examples from knowledge syntheses on interventions for bronchiolitis, a common acute paediatric condition. Some of the differences among these approaches are subtle and methods are not necessarily mutually exclusive to a single review type.
  
  
  CONCLUSIONS
  Systematic reviews bring together evidence from multiple studies in a rigorous fashion for a single intervention or group of interventions. Systematic reviews, as they have developed within healthcare, often focus on single or select interventions and direct pairwise comparisons; therefore, end-users may need to read several individual SRs to inform decision making. Overviews of reviews compile information from multiple SRs relevant to a single health problem. Overviews provide the end-user with a quick overview of the available evidence; however, overviews are dependent on the methods and decisions employed at the SR level. Furthermore, overviews do not often integrate evidence from different SRs quantitatively. Comparative effectiveness reviews, as we define them here, synthesize relevant evidence from individual studies to describe the relative benefits (or harms) of a range of interventions. Comparative effectiveness reviews may use statistical methods (network meta-analysis) to incorporate direct and indirect evidence; therefore, they can provide stronger inferences about the relative effectiveness (or safety) of interventions. While potentially more expensive and time-consuming to produce, a comparative effectiveness review provides a synthesis of a range of interventions for a given condition and the relative efficacy across interventions using consistent and standardized methodology.
authors.affiliation: Alberta Research Centre for Health Evidence, Department of Pediatrics, University of Alberta, Edmonton, Canada; Cochrane Child Health Field, Department of Pediatrics, University of Alberta, Edmonton, Canada. hartling@ualberta.ca.
authors.forename: Lisa
authors.initials: L
authors.lastname: Hartling
authors.forename: Ben
authors.initials: B
authors.lastname: Vandermeer
authors.forename: Ricardo M
authors.initials: RM
authors.lastname: Fernandes
mesh_terms: D001988:Bronchiolitis / Q000628:therapy; D002648:Child; D002669:Child Welfare; D057186:Comparative Effectiveness Research; D019317:Evidence-Based Medicine; D006801:Humans; D015201:Meta-Analysis as Topic; D012196:Review Literature as Topic
keywords: comparative effectiveness reviews; meta-analysis; mixed treatment comparison; network meta-analysis; overviews of reviews; systematic reviews