Your text discusses a case in which a researcher tries to test the effectiveness of a form of psychotherapy for depression by administering psychotherapy to depressed adolescents and seeing if they are still depressed 3 months later. Your book notes that many adolescents who are depressed at a given time will be happier later even without psychotherapy. For this reason, the passage of time is discussed as a potential source of:

a. internal invalidity
b. external invalidity
c. confusion over direction of causation
d. a sleeper effect

Ans: a. internal invalidity