the most credit is that in which discovery and explanation go hand in hand, in which not only are new facts presented, but their relation to old ones is pointed out” (Rayleigh, 1884). In other words, the contribution of any singular piece of research draws not only from its own unique discoveries, but also from its relationship to previous research (Glasziou et al., 2004; Mulrow and Lohr, 2001). Thus, the synthesis and assessment of a body of evidence is at the heart of a systematic review (SR) of comparative effectiveness research (CER).

The previous chapter described the considerable challenges involved in assembling all the individual studies that comprise current knowledge on the effectiveness of a healthcare intervention: the “body of evidence.” This chapter begins with the assumption that the body of evidence was identified in an optimal manner and that the risk of bias in each individual study was assessed appropriately—both according to the committee’s standards. This chapter addresses the synthesis and assessment of the collected evidence, focusing on those aspects that are most salient to setting standards. The science of SR is rapidly evolving; much has yet to be learned. The purpose of standards for evidence synthesis and assessment—as in other SR methods—is to set performance expectations and to promote accountability for meeting those expectations without stifling innovation in methods. Thus, the emphasis is not on specifying preferred technical methods, but rather the building blocks that help ensure objectivity, transparency, and scientific rigor.

As it did elsewhere in this report, the committee developed this chapter’s standards and elements of performance based on available evidence and expert guidance from the Agency for Healthcare Research and Quality (AHRQ) Effective Health Care Program, the Centre for Reviews and Dissemination (CRD, part of University of York, UK), and the Cochrane Collaboration (Chou et al., 2010; CRD, 2009; Deeks et al., 2008; Fu et al., 2010; Lefebvre et al., 2008; Owens et al., 2010). Guidance on assessing quality of evidence from the Grading of Recommendations Assessment, Development, and Evaluation (GRADE) Working Group was another key source of information (Guyatt et al. 2010; Schünemann et al., 2009). See Appendix F for a detailed summary of AHRQ, CRD, and Cochrane guidance for the assessment and synthesis of a body of evidence.

The committee had several opportunities for learning the perspectives of stakeholders on issues related to this chapter. SR experts and representatives from medical specialty associations, payers, and consumer groups provided both written responses to the committee’s questions and oral testimony in a public workshop (see Appendix C).

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