If a study has high internal validity but low external validity, which statement is true?

• A. The study's results will generalize well to real-world settings.
• B. Causal conclusions are invalid.
• C. Real-world applicability may be limited despite strong causal conclusions.
• D. The measurement is perfectly reliable.

Answer: (C)

The key idea here is the difference between internal and external validity. High internal validity means the study does a solid job of showing a cause-and-effect relationship within the study conditions and the sample studied. Low external validity means those findings may not apply to other people, settings, or times.

So, even with a clear causal link established inside the study, you’d expect real-world applicability to be limited because the results might not generalize beyond the specific context tested. That makes the statement that real-world applicability may be limited despite strong causal conclusions the best fit.

Why the other ideas aren’t correct: saying the results generalize well would require high external validity, which isn’t the case. Claiming causal conclusions are invalid would ignore the strong internal validity that supports the causal claim within the study. And claiming the measurement is perfectly reliable goes beyond what internal vs external validity tells us; reliability is about consistency of measurement and isn’t determined by the internal/external validity balance described here.