Which of the following is a factor that can threaten external validity?

Selection effects are a significant factor that can threaten external validity because they influence how generalizable the findings of a study are to a larger population. When a particular group of participants is chosen based on certain characteristics rather than through random sampling, the results may not be applicable to individuals outside that specific group. For example, if a study is conducted only on college students, the findings may not be relevant to older adults or those from different cultural backgrounds. This lack of diverse representation can skew the outcomes and limit the ability to apply the results to a broader audience.

In contrast, random selection of subjects helps to mitigate this threat by ensuring that every individual in the population has an equal chance of being included in the study, which aids in enhancing the generalizability of the results. Similarly, the use of control groups is designed to compare different interventions and ensure that any observed effects can be attributed to the treatment rather than external variables. Finally, while statistical significance indicates whether the results are likely due to chance, it does not directly relate to external validity, as a statistically significant finding may still lack broader applicability if the sample was not representative. Thus, understanding these concepts helps clarify the role of selection effects in affecting the external validity of research findings.