Impact evaluation is an assessment of how the intervention being evaluated affects outcomes, whether these effects are intended or unintended. The proper analysis of impact requires a counterfactual of what those outcomes would have been in the absence of the intervention.
There is an important distinction between monitoring outcomes, which is a description of the factual, and utilizing the counterfactual to attribute observed outcomes to the intervention. The IFAD impact evaluation guidelines accordingly define impact as the “the attainment of development goals of the project or program, or rather the contributions to their attainment.” The ADB guidelines state the same point as follows: “project impact evaluation establishes whether the intervention had a welfare effect on individuals, households, and communities, and whether this effect can be attributed to the concerned intervention”.
"Impact evaluation serves both objectives of evaluation: lesson-learning and accountability" OEDC
The most common counterfactual is to use a comparison group. The difference in outcomes between the beneficiaries of the intervention (the treatment group) and the comparison group, is a single difference measure of impact. This measure can suffer from various problems, so that a double difference, comparing the difference in the change in the outcome for treatment and comparison groups, is to be preferred.