Internal validity means the only thing that could have change the experiment is controlled for while external validity cannot be controlled because it’s outside the study. For example internal validity would be having a study to see does drinking effect how well people can walk on a line. The researcher would control how much alcohol each group gets and the type. An example of external validity would be having a study dealing with how well does a new sleeping medication work. A variable that cannot be controlled for is other factors at home affecting how participants sleeping, like parents of a new born.
