What is impact?

I once taught a workshop on impact evaluation methods to staff of a non-profit. I introduced the idea of impact, defining it as the change in outcomes that can be attributed to a program or intervention. This concept is at the heart of impact evaluation, which aims to measure the difference between the outcomes we observe with our program, and what would have happened if the program weren't there. One of the participants raised her hand, and insisted that it couldn't be an impact unless it was long-term, and holistic. I answered that ideally, impact should look at both the short-term and the long-term, across many outcomes, but that for the purpose of our lecture, it was also okay to refer to the "impact" a program has on a short-term outcome. For the remainder of the lecture, whenever I referred to impact, she would mutter "long-term". 

That was my first encounter with the difference between how "impact" is used by impact evaluators, and how the term is used in the traditional monitoring and evaluation (M&E) field. 

To get a sense of how traditional M&E folk view impact, here is the definition used by my talented colleague Dominique, who oversees monitoring and evaluation for the Ministry of Finance: 

"The long-term, cumulative effect of programs/interventions over time on what they ultimately aim to change, such as a change in HIV infection, AIDS-related morbidity and mortality. Note: Impacts at a population-level are rarely attributable to a single program/intervention, but a specific program/intervention may, together with other programs/interventions, contribute to impacts on a population."

Notice that Dominique's definition specifically points out the difficulty in attributing impact to specific interventions. This is pretty much in direct