Which statement best defines internal validity and external validity in program evaluation?

• A. Internal validity concerns generalizability to other settings; external validity concerns causality of observed effects.
• B. Internal validity concerns measurement accuracy; external validity concerns reliability.
• C. Internal validity concerns whether observed effects are due to the intervention; external validity concerns generalizability to other settings.
• D. Internal validity concerns sample size adequacy; external validity concerns participant adherence.

Answer: (C)

Internal validity looks at causation: are the observed effects truly caused by the intervention, or could other factors explain them? External validity is about generalizability: would the same results occur in other settings, with different people, or at different times. So the statement that best fits says internal validity concerns whether the observed effects come from the intervention, while external validity concerns generalizability to other settings. In practice, strong internal validity helps support a causal claim, and strong external validity helps you apply findings beyond the original study. The other ideas mix up different concepts—measurement accuracy or reliability, which are psychometric properties, or issues like sample size and adherence that affect power or implementation rather than the definitions of validity.