Which statement best describes external validity in a cohort study?

• A. Internal validity of the causal inference
• B. Precision of exposure measurement
• C. Consistency across subgroups within the study
• D. Generalizability of results to populations beyond the study, affected by selection of participants and setting

Answer: (D)

External validity is about whether the study’s findings can be applied to people and settings beyond those actually studied. In a cohort study, this generalizability hinges mainly on how participants were chosen and where the study took place. If the sample is representative of the target population and the setting resembles real-world conditions, the results are more likely to apply broadly. If the participants are highly selective or the setting is unusual, the ability to generalize is reduced.

Internal validity, by contrast, concerns whether the observed association reflects a causal effect within the study itself, not how widely the result can be applied. Precision of exposure measurement speaks to how accurately the exposure is measured, which affects reliability but not generalizability. Consistency across subgroups within the study relates to whether the finding holds across those subgroups and is more about internal consistency and effect modification within the study rather than generalizing to other populations.