What is External validity?
External validity is the extent to which study findings can be generalized to other populations, settings, times, and conditions beyond those specifically examined in the study.
How it works
Threats to external validity include: non-representative samples (studying only college students), artificial settings (lab vs. real world), temporal specificity (findings may not hold in a different era), and treatment variation (the specific implementation tested may differ from real-world delivery). External validity exists in tension with internal validity: tight experimental control increases confidence that the treatment caused the effect but decreases confidence that the effect generalizes.
Applied example
A medication found effective in a clinical trial of 25-45 year old men without comorbidities may have low external validity for elderly women with multiple health conditions, because the trial excluded the population most likely to need the medication.
Why it matters
External validity determines whether research discoveries become practical solutions or remain academic findings that do not transfer to real-world populations and contexts.
