Assessing impacts of public investments has long captured the interest and attention of the development community. This paper presents the evolution of different methods and approaches used for ex ante appraisal, monitoring, project evaluation, and impact assessment over the last five decades. Among these tools, impact assessment (IA) conducted retrospectively comes closest to providing the proof of development effectiveness. It is defined as the systematic analysis of the significant or lasting changes in people's lives brought about by a given action or series of actions in relation to a counterfactual. There are three basic types of retrospective IAs: macro-level IAs that focus on the contribution of developmental efforts to an impact goal aggregated at a sector or a system level; micro-level impact evaluations (IEs) concerned with estimating the average effect of an intervention on outcomes at the beneficiary level; and micro-level ex post impact analysis concerned with total effects of a development effort after the outputs are scaled-up. Ex post IAs have evolved and expanded over the decades in both breadth and depth of analysis in response to evolving development themes and methodological advancements. The increased emphasis on learning from evaluations has also seen responses from both quantitative and qualitative camps of the evaluation community. The paper argues that generation of robust knowledge that feeds into making developmental policies and investment decisions requires a hierarchical and cumulative approach to "improving the proof" through rigorous and a variety of impact assessment methods applied incrementally at the project, program and system level. Subjecting as many development interventions as resources allow to rigorous impact assessment based on a common framework can help build a critical body of evidence on impacts of development interventions, which can then be subjected to meta-analyses to help assimilate results across different studies and build a knowledge base on what works and what does not.

The world has made enormous progress in the past 50 years toward eliminating hunger and malnutrition. While, in 1960, roughly 30 percent of the world's population suffered from hunger and malnutrition, today less than 20 percent doessome five billion people now have enough food to live healthy, productive lives. Agricultural development has contributed significantly to these gains by increasing food supplies, reducing food prices, and creating new income and employment opportunities for some of the world's poorest people.This book examines where, why, and how past interventions in agricultural development have succeeded. It carefully reviews the policies, programs, and investments in agricultural development that have reduced hunger and poverty across Africa, Asia, and Latin America over the past half century. The 19 successes included here are described in in-depth case studies that synthesize the evidence on the intervention's impact on agricultural productivity and food security, evaluate the rigor with which the evidence was collected, and assess the tradeoffs inherent in each success. Together, these chapters provide evidence of "what works" in agricultural development.