External validity is the degree to which research findings can be generalised beyond the specific participants, settings, and conditions of the study. A study with high external validity produces results that hold across different populations, contexts, and times.
| Type | Question | SLA example |
|---|---|---|
| Population validity | Do findings apply to other learners? | Do results from university ESL students generalise to young EFL learners? |
| Ecological validity | Do findings apply in other settings? | Do lab results on Corrective Feedback hold in real classrooms? (See Ecological Validity) |
| Temporal validity | Do findings hold at other times? | Would a 1990s study on grammar instruction replicate in a 2020s communicative curriculum? |
| Treatment variation | Do findings hold with different implementations? | Does the effect of Focus on Form depend on the specific tasks used? |
Internal Validity and external validity often pull in opposite directions:
No single study can maximise both. This is why Replication across contexts and meta-analysis across studies matter: they build external validity cumulatively.