I understand the gist of all the definitions of internal and external validity I encountered to be the following:
- Internal validity is the extent to which confounding factors can be ruled out to cause the measured effect.
- External validity is the extent to which the results can be generalized to apply to other circumstances than those the study was conducted under.
However, if the measured effect varies with some circumstances, aren’t these circumstances confounding factors? If this were true, a study could not lack external validity without lacking internal validity.
I therefore find it hard to see why a threat to external validity wouldn't also be a threat to internal validity. Am I getting it wrong? I couldn't find any satisfying answer in the literature.