What is a confounder?

• A. An unrelated factor
• B. An outcome variable
• C. A variable related to exposure and outcome
• D. A variable that lies on the causal path between exposure and outcome

Answer: (C)

Confounding occurs when a third variable is related to both the exposure and the outcome, creating a distorted view of the exposure–outcome relationship. A confounder is precisely a variable that is associated with both exposure and outcome and is not on the causal path from exposure to outcome. This association can make it look like the exposure affects the outcome when the true driver is the confounder. For example, smoking is a confounder in studies of coffee consumption and heart disease because it tends to be higher among coffee drinkers and independently increases heart disease risk; if not adjusted, the link between coffee and heart disease may be exaggerated. The best answer describes a variable related to both exposure and outcome. An unrelated factor, the outcome itself, or a variable on the causal path (a mediator) do not fit the definition of a confounder.