Epidemiology is primarily focused on establishing valid associations between 'exposures' and health outcomes. However, establishing an association does not necessarily mean that the exposure is a cause of the outcome. Most definitions of "cause" include the notion that it is something that has an effect or a consequence. Certainly, establishing a valid association between exposure and outcome is a necessary first step that must be accomplished before wrestling with the more complicated, and frequently controversial, question of whether the relationship is causal. However, for most epidemiologists the second step in the process is to consider the entire body of evidence that is available to try to arrive at a reasonable conclusion about the relationship when the overall evidence from epidemiology and other sources (e.g., in vitro, animal, and other types of human studies) is reviewed.

Since a determination that a relationship is causal is a judgment, there is often disagreement, particularly since causality often implies some degree of responsibility for the outcome or may create a demand for public health action, and this often has legal and financial consequences. Many would agree that incomplete evidence or a lack of agreement about causality should not always prevent appropriate actions to protect the public's health. Nevertheless, the question of whether a relationship is causal sometimes has important consequences for a vast number of people. 

 "The world is richer in associations than meanings, and it is the part of wisdom to differentiate the two."

— John Barth, novelist.

Learning Objectives

After completing this module, the student will be able to: