Impact evaluations can be used either to estimate the impact of an entire program or to evaluate the effect of a new product or policy. In either case, the fundamental evaluation question is the same: "How are the lives of the participants different relative to how they would have been had the program, product, service, or policy not been implemented?" The first part of that question, how are the lives of the participants different, is the easy part. The second part, however, is not. It requires measuring the counterfactual, how their lives would have been had the policy not been implemented. This is the evaluation challenge. One critical difference between a reliable and unreliable evaluation is how well the design allows the researcher to measure this counterfactual.