Which statement best differentiates internal validity from external validity in social research?

Which statement best differentiates internal validity from external validity in social research?

• A. Internal validity is about sampling bias; external validity concerns measurement error.
• B. Internal validity equals reliability; external equals validity.
• C. Internal validity refers to confidence that the observed effect is caused by the manipulation; external validity refers to generalizability.
• D. Internal validity concerns generalizability to other settings; external concerns causal inference.

Answer: (C)