Confounding variable in epidemiology is defined as what?

• A. A variable that is unrelated to exposure and outcome
• B. A variable that is a mediator between exposure and outcome
• C. A variable that modifies the effect of exposure on the outcome
• D. A variable that is a common cause of both exposure and outcome and distorts their observed association

Answer: (D)

Confounding occurs when a third factor is related to both the exposure and the outcome, and it is not on the causal path between them. This means the observed link between exposure and outcome can be distorted because the confounder creates a false impression of association (either exaggerating or masking the true effect). The confounding variable acts as a common cause of both exposure and outcome, which is why it can bias results if not controlled. It’s not a mediator, which would lie on the causal pathway from exposure to outcome, nor an effect modifier, which changes the strength of the association across its levels. To address confounding, researchers use methods like randomization, restricting groups, matching, or statistical adjustment to account for the confounder. For example, if age is related to both how much someone exercises and their risk of heart disease, age can confound the exercise–heart disease relationship unless it’s controlled for.