External validity concerns generalizability, and its first requirement is which of the following?

• A. Internal validity must be present.
• B. Large sample size guarantees generalizability.
• C. External validity is independent of study design.
• D. Blinding of outcome assessment is required.

Answer: (A)

Generalizability rests on external validity, but the essential first requirement is that the study be internally valid. Internal validity means the study design and conduct properly establish the relationship of interest within the study sample, protecting against bias, confounding, and measurement error. If internal validity is compromised, the observed effect may be distorted, so any attempt to generalize those findings to other populations or settings would be inappropriate, regardless of sample size or setting. A large sample size improves precision but does not fix bias, so it does not by itself guarantee generalizability. External validity is influenced by study design and how representative the sample is, but you need trustworthy, internally valid results before considering how well they apply elsewhere.