Which sampling approach most directly threatens external validity when the sample is drawn only from a single university campus?

• A. Simple random sampling
• B. Stratified sampling
• C. Convenience sampling
• D. Cluster sampling

Answer: (C)

External validity hinges on how well findings generalize beyond the study sample. If you only sample from a single university campus, the pool of participants is already limited to that campus's characteristics, environment, and student body. The approach that most directly worsens generalizability in this situation is convenience sampling—choosing participants simply because they are easy to recruit. This method tends to produce a biased group, overrepresenting people who are readily available or willing to participate and underrepresenting others who differ in meaningful ways. That strong bias makes it much harder to apply results to other campuses, other universities, or broader populations.

Other methods aim to reduce bias within the campus frame. Simple random sampling gives each student on the campus a fair chance to be included, which helps with representativeness within that setting. Stratified sampling ensures that important subgroups on the campus are represented in the sample, and cluster sampling can be efficient while still working within the campus structure. Although all of these still limit generalization beyond the campus, they don’t introduce the same level of unrepresentativeness that comes with convenience sampling.