define: validity of study
External validity refers to the extent to which the results of a study can be generalized to other settings (ecological validity), other people (population validity) and over time (historical validity). External validity can be improved by setting experiments in a more natural setting and using random sampling to select participants.
Internal validity is the extent to which the results of a study are true. That is, the intervention really did cause the change in behavior. The change was not the result of some other extraneous factor, such as differences in assessment procedures between intervention and control participants.