Impact heterogeneity refers to differences in impact due by beneficiary type and context.
In experimental evaluations the comparison group is called a control group. For instance, in a 5-year agricultural project, seeds are inputs, farmers trained in using them our outputs, changes in crop yields as a result of the seeds being planted properly in an outcome and families being more sustainably food secure over time is an impact.
It is safe to say that if an inadequate design yields bias, the stakeholders who are largely responsible for the funding of the program will be the ones most concerned; the results of the evaluation help the stakeholders decide whether or not to continue funding the program because the final decision lies with the funders and the sponsors.
Intervention interrupted time-series ITS evaluations require multiple data points on treated individuals before and after the intervention, while before versus after or pre-test post-test designs simply require a single data point before and after.
For instance, most Monitoring and Evaluation 'logical framework' plans have inputs-outputs-outcomes and Systematic reviews may also involve the synthesis of qualitative information, for example relating to the barriers to, or facilitators of, intervention effectiveness.
White b  advocates more widespread application of a theory-based approach to impact evaluation as a means to improve policy relevance of impact evaluations, outlining six key principles of the theory-based approach: Map out the causal chain program theory which explains how the intervention is expected to lead to the intended outcomes, and collect data to test the underlying assumptions of the causal links.