The process in which a treatment is administered might change the way people behave, changing outcomes. Trials with control groups are easier to justify as havi

The process in which a treatment is administered might change the way people behave, changing outcomes. Trials with control groups are easier to justify as having avoided these issues. If you're interested in learning more, I would recommend learning about Judea Pearl's do-calculus, specifically the backdoor criterion. Although it isn't the end-all of causal analysis, it is a very useful non-experimental approach that will help you develop an appreciation for the benefits of controlled experiments. As far as seat belts go, that's actually been something discussed a bit in economics. The main concern is if adding seat belts changes how people drive (confounding the actual effects of seat belts). The solution was to measure the protective effect of seat belts on occupants of the passenger seat in collisions involving drunk drivers.