External validity is a concept in research that refers to the extent to which the results of a study can be generalized to other situations, people, stimuli, and times. In other words, it is the validity of applying the conclusions of a scientific study outside the context of that study. External validity is an important property of any study because general conclusions are almost always a goal in research. However, note that a studys external validity is limited by its internal validity. If a causal inference made within a study is invalid, then generalizations of that inference to other contexts will also be invalid.

There are two types of external validity: population validity and ecological validity. Population validity refers to the extent to which the results of a study can be generalized to other populations, while ecological validity refers to the extent to which the results of a study can be generalized to other settings or situations.

In qualitative research, external validity is replaced by the concept of transferability, which is the ability of research results to transfer to situations with similar parameters, populations, and characteristics.

Threats to external validity include selection bias, history, maturation, testing, instrumentation, regression to the mean, and experimenter effects. Researchers can improve external validity by using field experiments, increasing the sample size, using random sampling, and using multiple measures.