External validity is a concept in psychology that refers to the extent to which the results of a study can be generalized to other contexts, situations, individuals, or populations. It is an important property of any study because general conclusions are almost always a goal in research. External validity is different from internal validity, which refers to the degree to which a study’s results reflect the true cause-effect relationships within the study.

There are two main types of external validity: population validity and ecological validity. Population validity refers to whether the findings from a sample can be reasonably generalized to a larger group of people (the population) . Ecological validity, on the other hand, refers to whether the study’s results can be generalized to other settings or populations.

There are several threats to external validity, including selection bias, history, experimenter effect, Hawthorne effect, testing effect, aptitude-treatment, and situation effect. To counter these threats, researchers can use random sampling, replication, and field experiments.

In summary, external validity is a crucial concept in psychology that helps researchers determine how trustworthy and meaningful the results of a study are. It is concerned with the extent to which the results of a study can be generalized to other contexts, situations, individuals, or populations.