Which statement best differentiates internal validity from external validity in program evaluation?

• A. Internal validity concerns causality; External validity concerns generalizability.
• B. Internal validity concerns generalizability; External validity concerns causality.
• C. Internal validity concerns measurement reliability; External concerns validity.
• D. Internal validity concerns sample size; External validity about ethics.

Answer: (C)

The main idea is distinguishing whether a study shows that the program caused observed changes within the study, versus whether those changes would apply beyond the study to other people, places, or times. Internal validity is about causality—can we attribute the observed effect to the program itself rather than other confounding factors? External validity is about generalizability—would the same effect occur in different settings or with different groups outside the study?

Reliability of measurements matters for trustworthiness, but it isn’t the core distinction between these two validity types. So the statement that internal validity is about causality and external validity is about generalizability best captures the difference.