 As discussed in  Section 5,  RCTs and many observational studies are population-level studies, which produce average results that are not straightforwardly applicable to individuals. If, say, treatment X reduces the risk of suffering from some adverse event over a period of time by 50% in population p, that is, the risk ratio (RR) for this treatment is 50%, then there may be no individual in p for whom the treatment halves the risk. Instead, the RR may vary dramatically among the subpopulations of p, and it may well be the case that Y is more effective than X for some subpopulations.