the primary goal of running an experiment is to obtain causal leverage, and it does so by randomly assigning subjects to experimental conditions, which sets it apart from observational studies. in an experiment with perfect compliance, the average treatment effect can be obtained easily. however, many experiments are likely to experience either one-sided or two-sided non-compliance. in the presence of non-compliance, the ate can no longer be recovered. instead, what is recovered is the average treatment effect for a certain subpopulation known as the compliers, which is the late.