Dr. Claussen conducts research on early intervention programs for children and adolescents who are at risk of substance use disorders. All of this research has been conducted in a university setting under tightly controlled conditions with highly trained and well-supervised staff. Dr. Claussen is concerned about how well the research will generalize to the community and whether the intervention will be as successful in more diverse populations. What is Dr. Claussen concerned about? 

a. Test-retest reliability 

b. External validity 

c. Interrater reliability

d. Internal validity