One of the main goals of epidemiology is to identify causal relationships between outcomes - like death, diseases, or injuries - and exposures - like smoking cigarettes, eating junk food, or drinking alcohol.
For example, nowadays, it’s widely known that smoking cigarettes causes lung cancer, or in other words, that smoking cigarettes leads to the development of lung cancer in many people. But how did we figure this out?
In the mid- 1950’s, an epidemiologist named Sir Austin Bradford Hill came up with nine guidelines for determining whether or not two things are causally related, and these are called the Bradford Hill Criteria.
The first criterion refers to the strength of association, and says that two things are more likely to be causally related if the strength of association between them is large.
For example, for people who smoke, the relative risk of lung cancer is around 25, meaning people that smoke have 25 times the risk of developing lung cancer compared to people that don’t smoke.
On the flip side, the relative risk of breast cancer for people who smoke is only about 1.5, which is much lower than the relative risk of lung cancer.
So, smoking is much more likely to cause lung cancer than it is to cause breast cancer.
The second criterion states that causal relationships are dose- dependent, meaning a person who has higher amounts of exposure will also have a higher risk of developing the outcome.
For example, the relative risk of lung cancer for people who smoke 10 cigarettes per day might be 8, the relative risk of lung cancer for people who smoke 15 cigarettes per day might be 13, and the relative risk of lung cancer for people who smoke 20 cigarettes per day might be 22.
