Generalizability refers to the extent to which findings in a particular study can be applied or extended to populations beyond the population studied. The differences in populations could be age, sex, gender, health status, race, ethnicity, geography, marital status or any number of other possible ways to categorize populations.
Deeper dive
For example, extremely few studies on children could be generalized to adults and vice versa. This age restriction is built into the FDA drug approval process: a drug cannot be approved for age groups that were not studied in the clinical trials because there is no evidence that the safety and effectiveness could be generalized to groups outside the age ranges initially studied. Differences can be far more subtle, however. For example, the findings of a study of transitioned transgender children’s mental health in the greater Seattle area may not be able to be generalized to the mental health of their otherwise identical counterparts in rural Mississippi because the social acceptance and cultural environment of those different geographical areas could so greatly influence the mental health of this population. Similarly, findings in a group of patients with a specific condition, such as high blood pressure, cannot be presumed to apply to patients without that condition unless a different study provides evidence for it. Any characteristic that could differ across populations could be a barrier to generalizability of findings in both clinical trials and in observational studies.