External validity
Sigmund freud's psychoanalytic theory was based primarily on descriptive data from a few case studies. Critics question how well the experiences of these unusual individuals represent others. The concern referred to is that this type of research may lack external validity.
External validity is the extent to which the information and theories of a study can be generalized to other conditions or to other people. People, places and times are the three major threats ( factors that could be wrong) to external validity .