Systematic reviews are central to scientific inquiry into what is known and not known about what works in health care (Glasziou and Haynes, 2005; Helfand, 2005; Mulrow and Lohr, 2001; Steinberg and Luce, 2005). In 1884, J. W. Strutt Lord Rayleigh, who later won a Nobel prize in physics, observed that the synthesis and explanation of past discoveries are integral to future progress (Rayleigh, 1884). Yet, more than a century later, Antman and colleagues (1992) and Lau and colleagues (1992) clearly demonstrated that this message was still largely ignored, with the potential for great harm to patients. In a series of meta-analyses examining the treatment of myocardial infarction, the researchers concluded that clinicians need better access to syntheses of the results of existing studies to formulate clinical recommendations. Today, systematic reviews of the available evidence remain an often undervalued scientific discipline.

This chapter has three principal objectives: (1) to describe the fundamental components of a systematic review, (2) to present the committee’s recommendations for conducting systematic evidence reviews under the aegis of a proposed national clinical effectiveness assessment program (“the Program”), and (3) to highlight the key challenges in producing high-quality systematic reviews.

A systematic review is a scientific investigation that focuses on a specific question and uses explicit, preplanned scientific methods to identify, select, assess, and summarize similar but separate studies (Haynes et al., 2006; West et al., 2002). It may or may not include a quantitative synthesis of the results from separate studies (meta-analysis). A meta-analysis quantitatively combines the results of similar studies in an attempt to allow inference from the sample of studies included to the population of interest. This report uses the term “systematic review” to describe reviews that incorporate meta-analyses as well as reviews that present the study data descriptively rather than inferentially.