Complex interventions present unique challenges for systematic reviews. Current debates tend to center around describing complexity, rather than providing guidance on what to do about it. At a series of meetings during 2009-2012, we met to review the challenges and practical steps reviewer could take to incorporate a complexity perspective into systematic reviews. Based on this, we outline a pragmatic approach to dealing with complexity, beginning, as for any review, with clearly defining the research question(s). We argue that reviews of complex interventions can themselves be simple or complex, depending on the question to be answered. In systematic reviews and evaluations of complex interventions, it will be helpful to start by identifying the sources of complexity, then mapping aspects of complexity in the intervention onto the appropriate sources of evidence (such as specific types of quantitative or qualitative study). Although we focus on systematic reviews, the general approach is also applicable to primary research that is aimed at evaluating complex interventions. Although the examples are drawn from health care, the approach may also be applied to other sectors (e.g., social policy or international development). We end by concluding that systematic reviews should follow the principle of Occam's razor: explanations should be as complex as they need to be and no more.
Keywords: Complex interventions; Complexity; Evaluation; Evidence synthesis; Qualitative research; Systematic reviews.
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