Astronomy for development projects conceive of development in very broad terms and seek to affect a wide range of social outcomes. The histories of education, development economics and science communication research indicate that positive social impacts are often difficult to achieve. Without a scientific approach, astronomy's potential as a tool for development may never be realised nor recognised. Evidence-informed project design increases the chances of a project's success and likely impact while reducing the risk of unintended negative outcomes. The IAU Office of Astronomy for Development (OAD) Impact Cycle is presented here as a possible framework for integrating evaluation and evidence-based practice in global Astronomy outreach and education delivery. The suggested framework offers a way to gradually accumulate knowledge about which approaches are effective and which are not, enabling the astronomy community to gradually increase its social impact by building on its successes.