What is selection bias in a cohort study and how can it arise?

• A. Selection bias occurs when the study uses random sampling.
• B. Selection bias arises when participants drop out at random.
• C. Selection bias arises when exposure status is misclassified.
• D. Selection bias arises when the exposed and unexposed groups differ in ways related to the outcome due to nonrandom selection.

Answer: (D)

Selection bias in a cohort study happens when the way participants are chosen into the exposed and unexposed groups creates systematic differences between those groups that are related to the outcome. Because the groups are formed through nonrandom selection, they may differ in factors other than the exposure that also influence the outcome. This means the observed association can reflect these preexisting differences rather than a true effect of the exposure. For example, if healthier, more health‑minded individuals are more likely to enroll in the exposed group, their better outcomes might be due to those baseline characteristics rather than the exposure itself. Random sampling helps reduce this bias, while dropout occurring at random isn’t a source of selection bias, and misclassification of exposure is a different type of bias (information/measurement bias).