Association versus Causation
When considering the relationship between exposures and health outcomes, it is important to distinguish between association and causation. Epidemiologists ultimately want to be able to draw conclusions about causation, but most epidemiologic studies focus on establishing associations.
Association: Is a specified health outcome more likely in people with a particular "exposure"? Is there a link? Association is a statistical relationship between two variables.
Two variables may be associated without a causal relationship. For example, there is a statistical association between the number of people who drowned by falling into a pool and the number of films Nicolas Cage appeared in in a given year. However, there is obviously no causal relationship.
Jewish women have a higher risk of breast cancer, while Mormons have a lower risk. However, one's religion is not a cause of breast cancer. There are other explanations.
It has been convincingly demonstrated that people of lower socioeconomic status (SES) have a higher risk of lung cancer, i.e., there is a clear association, but does that mean that low SES is a cause of lung cancer? A more plausible explanation is that people of lower SES are more likely to smoke and to be chronically exposed to air pollution and that exposure of the respiratory tract to these contaminants causes mutations in bronchial cells that can eventually produce a cancer.
Causation: Causation means that the exposure produces the effect. It can be the presence of an adverse exposure, e.g., increased risks from working in a coal mine, using illicit drugs, or breathing in second hand smoke. Causative factors can also be the absence of a preventive exposure, such as not wearing a seatbelt or not exercising. A cause must be associated with the outcome, but simply demonstrating an association is not enough. To conclude that lack of exercise is a cause of heart disease, one needs to review the body of evidence suggesting a causal relationship and also consider other criteria. For example,
- Does exposure to tobacco smoke and air pollution precede the occurrence of lung cancer?
- Is there a strong association between smoking and subsequent occurrence of lung cancer?
- Is it possible that having lung cancer causes one to smoke?
It is interesting to note that when lifelong smokers are told they have lung cancer or emphysema, many them quit smoking. This makes it seem as if ex-smokers are more likely to die of emphysema or lung cancer than current smokers.