<Li> Pre-test effects: If cause - effect relationships can only be found when pre-tests are carried out, then this also limits the generality of the findings . </Li> <Li> Post-test effects: If cause - effect relationships can only be found when post-tests are carried out, then this also limits the generality of the findings . </Li> <P> Note that a study's 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 . </P> <P> Cook and Campbell made the crucial distinction between generalizing to some population and generalizing across subpopulations defined by different levels of some background factor . Lynch has argued that it is almost never possible to generalize to meaningful populations except as a snapshot of history, but it is possible to test the degree to which the effect of some cause on some dependent variable generalizes across subpopulations that vary in some background factor . That requires a test of whether the treatment effect being investigated is moderated by interactions with one or more background factors . </P>

External validity is the extent to which one factor truly causes another factor