When two variables are correlated, one variable may or may not be causing the other

What factor is the strongest predictor of teenage substance use?

A) whether or not your parents use substances B) having friends who use substances C) doing poorly in school D) having a sensation-seeking personality

Dr. Wegner conducts a study to investigate the careers young adults are interested in pursuing using a sample of 100 Harvard honors students. What is the major problem with her sample?

a. It lacks internal validity. b. It is unlikely that she can generalize across situations. c. It is difficult to say with certainty that she can generalize across people. d. It is too small in size to provide any meaningful information.