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.

venn diagram showing relationship of exposure to outcome

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.

Graph with two lines, one showing the number of people drowning by falling into a pool each year with numbers that vary from 90 to 125 per year. The second line shows the number of films that Nicolas Cage appeared in during each of those years, varying between one to four. Although the two phenomena are on different scales they appear to fluctuate in a highly correlated way

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,

It is interesting to note that when lifelong smokers are told they have lung cancer or emphysema, many of them quit smoking. This makes it seem as if ex-smokers are more likely to die of emphysema or lung cancer than current smokers.