External validity

External validity concerns whether a result would still apply outside the exact circumstances of the study that produced it: different patients, clinicians, settings, or eras. It is separate from internal validity, which is about whether the study measured its own effect without bias. A tightly controlled trial can have strong internal validity but limited external validity if, for example, it enrolled only younger participants without other illnesses. When reading a study, compare the enrolled population and setting against the group you are actually interested in, and check eligibility criteria, dropout, and how closely the intervention resembles real-world use. Ask what features might make the effect larger or smaller elsewhere. Example: a drug trial that excluded people over 75 and those with kidney disease may report a clear benefit, yet leave genuinely uncertain whether that benefit extends to an older, sicker population treated in ordinary practice.