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Theory-based systematic reviews, which summarise evidence on what works, when and why, strive for greater policy relevance. Reviews that answer these questions adopt a mixed methods approach and draw on a range of study types. Answering the ‘what works’ and ‘what doesn't’ questions means drawing on effectiveness studies, conducted to standards of high-quality impact evaluation. But in formulating answers to the ‘when’ and ‘why’ questions requires a broader range of evidence from both quantitative and qualitative research. Based on a review of the methodological literature in this field and the experience of 3ie's systematic reviews program, this article provides an outline of how a theory-based approach to systematic reviews, including appropriate quantitative and qualitative evidence, can be operationalised. We propose an approach based on three principles: (1) develop a program theory for the intervention; (2) adopt a mixed methods approach and include a broader range of evidence; and (3) maintain the rigour and transparency that characterise systematic reviews. The approach translates into two broad options. Effectiveness plus reviews focus on providing a detailed causal chain analysis by drawing on a program theory and additional data collection on context and intervention implementation. And effectiveness plus with a parallel review component, which is designed to answer specific research questions related to effectiveness, adopts separate inclusion criteria, reflecting the type of studies appropriate for answering those questions.
Birte Snilstveit (Mon,) studied this question.