Which design choice improves external validity?

• A. Random assignment is the primary method to improve external validity.
• B. Increasing the sample size alone improves external validity.
• C. Random sampling from the population improves external validity.
• D. Double-blind designs increase external validity.

Answer: (C)

External validity is about whether study results generalize beyond the participants in the study to the wider population. The best way to boost this generalizability is to sample randomly from the population, giving each individual an equal chance to be included. Random sampling helps create a sample that reflects the population’s diversity and characteristics, so the observed effects are more likely to hold true in real-world settings and with different people.

Random assignment, on the other hand, mainly strengthens internal validity by ensuring groups are comparable, which supports causal conclusions within the study but doesn’t by itself guarantee generalizability to the population. Simply increasing the sample size improves precision within the sample but won’t fix a biased sample. Double-blind designs reduce bias in measurement and expectations inside the study, again strengthening internal validity rather than external generalizability.