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.

Prospects for Livestock-Based Livelihoods in Africa's Drylands examines the challenges and opportunities facing the livestock sector and the people who depend on livestock in the dryland regions of Sub-Saharan Africa. It presents a novel way of thinking about pastoral development, grounded in a conceptual framework that focuses on the multiple shocks that drylands livestock keepers face and how those shocks can be addressed, drawing on a state-of-the-art literature review carried out by scientists of leading research institutes and development organizations, and integrating the results of an innovative approach to modeling development options for the drylands livestock sector. Looking to the future, the picture is mixed. On the positive side, demand for red meat is expected to strengthen in domestic and regional markets, suggesting that livestock keepers will have good market opportunities. On the negative side, a large majority of livestock keepers are classifi ed as poor, and the natural (feed) resource base is likely to be suffi cient to enable improved meat and milk production for the growing human population. Prospects for the livestock sector through 2030 vary by aridity zone. In arid and semi-arid zones, a reasonable goal for 2030 is to have land use, training, and microfi nance systems established that promote an appropriate balance between human and livestock carrying capacities, featuring mainly grassland/pastoral systems that reliably and sustainably satisfy the minimum income needs of herder households, produce at least a signifi cant part of the demand in local markets for animal source food, and provide environmental services for which livestock keepers receive compensation. The goal includes signifi cant employment generation outside the sector. In the higher rainfall zones of the semi-arid areas, and in the subhumid zones, a reasonable goal for 2030 is to have intensifi ed production systems established, featuring mainly mixed livestock/arable farming or agro-pastoral systems that are closely linked to nearby grassland/pastoral systems and that consistently generate marketable surpluses of differentiated red meat and livestock products that can compete not only in the expanding domestic market but also in selected regional markets.