Distinguish confounding from effect modification.

• A. Confounding is a real variation; effect modification is bias
• B. Effect modification is a real variation in effect by levels of another variable; confounding is a bias
• C. They are the same concept
• D. Confounding occurs only in randomized trials

Answer: (B)

Confounding and effect modification describe two different things about how a third variable relates to exposure and outcome. Confounding happens when a third factor is associated with both the exposure and the outcome and this association distorts the observed relationship, creating a biased estimate that does not reflect the true effect. Effect modification, or interaction, occurs when the exposure’s effect on the outcome genuinely differs across levels of a third variable; this is a real variation in effect, not a bias. A helpful way to keep them straight is: confounding makes you wrong about the size or direction of the effect, while effect modification shows that the effect is different in different subgroups and should be described separately for those groups. For example, if a drug’s benefit varies by age, that’s effect modification. If older people are more likely to receive the drug and also have a higher baseline risk independent of the drug, age is a confounder producing a biased estimate unless you adjust for it. In practice, confounding is addressed through design or analytic adjustment to isolate the true effect, whereas effect modification is reported as subgroup-specific effects or tested as an interaction.