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This slide show by Dr Howard White presents the use of randomized control trials (RCTs) in impact evaluation.
It draws on examples of impact evaluations that were done by the International Initiative for Impact Evaluation (3ie) and explains the process used for randomized sample selection.
Sources
International Fund for Agricultural Development (IFAD) (2012). Rigorous impact evaluations are much more than just evaluations. IFAD Social Reporting Blog, Posted by Roxanna Samii Thursday, September 20, 2012, at http://ifad-un.blogspot.com.ar/2012/09/rigorous-impact-evaluations-are-much_20.html