It is already known in the world of public polices and social projects' interventions that knowing if pre-defined objectives have been attained is a difficult tasa. But even more difficult is to know if those polices and interventions have had an impact in the aimed population.
The newest technique developed by economists are Randomized Controlled Trials (RCT). These trials do not simplify the process nor they make it cheaper, nonetheless they are the most accurate technique science has yet discovered. They allow to know not only the results attained but also the impact achieved by an intervention.
The published article in Effective Philanthropy talks about the resistente showed by the sector regarding RCTs, and underlines the fact that even though the resistance is explained by practical difficulties in applying RCTs, the bottom problem are different points of view of the Theory of Change.