According to this Hierarchy of Evidence, most epidemiologic studies fall in the lower half. That’s because the research that Public Health practitioners, as epidemiologists can conduct are usually observational in nature. As such, these researchers do not have the control that would enable them to truly say that “A causes B.” What they can say, however, is “A may cause B.”

This is the reason why epidemiologic studies need to have good research design that would allow replication so that results from many studies can build up an “overwhelming” body of evidence that the associations found can be considered causal associations.