The validity of a research tool tells us how accurately it measures what it is supposed to measure. The internal validity of a study is important when assessing causation. Internal validity refers to whether the observed effects of a variable under investigation can be attributed to the hypothesized cause. There are many threats to internal validity which are generally related to whether there is a plausible alternative for the observed effects.
Unlike internal validity, which is only related to a specific study, external validity is related to whether the results of research can be generalized to a larger population. External validity is related to sampling design and assumptions made about the relationship between the sample and general population.