This has the advantage of allowing us to look at lifelong exposures. It has the disadvantage of not being controlled, as is a laboratory experiment. The science of epidemiology uses statistics to look for apparent connections between causes and effects. No , it doesn't. Since everyone gets sick, it is not enough to observe a group with a high incidence of a given disease and then assume that a common trait of that group causes the disease.
Statistics are extremely powerful and can be dangerous.
It is very easy to use numbers to show just about anything. However, even this does not tell the whole story. If a difference is found, it could be due to a common trait of the group, but it also might be another, less evident factor. I'll discuss more about epidemiology in the coming months. Epidemiology is very useful for suggesting possible links between cause and effect.