Which programs work and what impacts they achieve are questions of key relevance to development professionals, policy-makers as well as donors. By providing specific answers to these questions, rigorous impact evaluations offer valuable guidance. Impact evaluations furthermore increase accountability towards stakeholders and funders, be they private individuals or firms, organizations, foundations or taxpayers, analyzing value-for-money and cost-effectiveness.
Impact evaluations aim at measuring the real changes achieved through specific policies, programs, products or interventions. They go beyond mere accounting of inputs (e.g., amount of financial resources spent) and outputs (e.g. number of programs delivered), and rather ask the questions: “Did the program really help? Did it change the situation to the better as compared to the status quo? How could the program be further improved?” For instance, instead of simply answering how many children have been reached by an educational program, impact evaluations measure by how much children’s educational outcomes (knowledge and skills) have improved because of the program.
Essentially, impact evaluations require comparing the situation observed after the program is implemented with what would have happened had the program not been implemented (the so-called counterfactual). This is why impact evaluations use control groups, which are groups of individuals which did not participate in the program but are otherwise comparable. Comparable control groups enable constructing an estimate of the counterfactual outcome of what would have happened to the participants if they had not participated. The comparability of the control groups is of central importance for the credibility of the evaluation.