The problem with epidemiological studies is that you can t, except in a few instances, do clinically controlled experiments, since they would knowingly expose people to a potentially dangerous element. For example, if you wanted to judge how dangerous radon is, you couldn t expose people to varying amounts of radon to see how many get cancer. A cohort study begins with a group of people who do not have the disease and follows them, measuring suspected characteristics and health performance. Periodic blood tests were taken to measure lead exposure and these were correlated with IQ test performance. Cohort studies take a lot of time and are very expensive.
Cohort studies are the best epidemiological studies.
They try to keep age, sex, socio-economic status equal. The researchers try to find some key element in the histories of the two groups that might explain the disease. Case-control studies are weaker than cohort studies. Rather than dealing with individuals, these studies look at groups one neighborhood vs. another neighborhood, blacks vs. whites, etc. These studies are very crude.