We provide a 'how to' guide to undertake systematic reviews of effects in international development, by which we mean, synthesis of literature relating to the effectiveness of particular development interventions. Our remit includes determining the review's questions and scope, literature search, critical appraisal, methods of synthesis including meta-analysis, and assessing the extent to which generalisable conclusions can be drawn using a theory-based approach. Our work draws on the experiences of the International Initiative for Impact Evaluation's (3ie's) systematic reviews programme.

The Campbell Collaboration (C2) was founded on the principle that systematic reviews on the effects of interventions will inform and help improve policy and services. C2 offers editorial and methodological support to review authors throughout the process of producing a systematic review. A number of C2's editors, librarians, methodologists and external peerreviewers contribute. The Campbell Collaboration

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We conclude that tools for nonrandomized studies of interventions need to be further developed to incorporate evaluation questions for quasi-experiments with selection on unobservables.

Contributions Jos Vaessen (JV), Maren Duvendack (MD), Hugh Waddington (HW) and FransLeeuw (FL) contributed to the writing and revising of this systematic review. JV, Ana Rivas and Ger van Gils (GG) contributed to the design of the review and the information retrieval activities associated with the review. In the quality assessment of selected studies, we distinguish between assessment of methodological quality, carried out by MD, Richard Palmer Jones (RPJ) and HW, and assessment of the quality of the theoretical framework of selected studies carried out by GG and FL. The meta-analysis was conducted by MD, RPJ and HW, with additional support from Jorge Hombrados. The qualitative synthesis was conducted by GG, FL and JV. Nathalie Holvoet and Johan Bastiaensen helped developing the theoretical framework on microcredit and empowerment. Ruslan Lukach contributed to methodological design. JV will be responsible for updating this review as additional evidence accumulates and as funding becomes available.