 There are in fact two problems of external validity in the application of RCT results. On the one hand there is the population-level problem of making an inference from test to target population. On the other hand, there is the problem of making an inference from population to individual. The RCT provides evidence for a population-level claim: “In population p (the test population) intervention X is effective in the treatment of condition Y”.  For this claim to be true, the treatment must be on average effective, which allows the effectiveness to vary among the individuals in the population. Indeed, it is possible that the intervention is effective (and beneficial) on average but ineffective or positively harmful in some individuals (i.e., members of some subpopulations). Proponents of EBM to some extent oversell their case when they write that EBM 