External Validity: The Next Step for Systematic Reviews?

Background: Systematic reviews—which identify, assess, and summarize existing research—are usually designed to determine whether research shows that an intervention has evidence of effectiveness, rather than whether an intervention will work under different circumstances. The reviews typically focus on the internal validity of the research and do not consistently incorporate information on external validity into their conclusions. Objectives: In this article, we focus on how systematic reviews address external validity. Methods: We conducted a brief scan of 19 systematic reviews and a more in-depth examination of information presented in a systematic review of home visiting research. Results: We found that many reviews do not provide information on generalizability, such as statistical representativeness, but focus on factors likely to increase heterogeneity (e.g., numbers of studies or settings) and report on context. The latter may help users decide whether the research characteristics—such as sample demographics or settings—are similar to their own. However, we found that differences in reporting, such as which variables are included and how they are measured, make it difficult to summarize across studies or make basic determinations of sample characteristics, such as whether the majority of a sample was unemployed or married. Conclusion: Evaluation research and systematic reviews would benefit from reporting guidelines for external validity to ensure that key information is reported across studies.