Plain Language Summaries: A Systematic Review of Theory, Guidelines, and Empirical Research

Plain language summaries (PLS) have been introduced to communicate research in an understandable way to a non-expert audience. Guidelines for writing PLS have been developed and empirical research on PLS has been conducted, but terminology and research approaches in this comparatively young field vary considerably. This prompted us to review the current state of the art on the theoretical and empirical literature on PLS. Main objectives were (1) to develop a conceptual framework for PLS theory, and (2) to synthesize empirical evidence on PLS. The major research questions were how theoretical considerations on PLS can be conceptualized and what the empirical evidence of their effects on defined outcomes is. Methods: Web of Science, PubMed, PsycInfo, and PSYNDEX were searched in July 2020. We included empirical investigations of PLS, reports on development or evaluation of PLS, PLS guidelines and theoretical articles referring to PLS for further analysis. Through content analysis, a conceptual framework was developed and empirical studies investigating the effects of PLS on defined outcomes were narratively synthesized. Results: We identified 5,481 records, of which 72 articles met the inclusion criteria. All articles were used to develop a conceptual framework for PLS which comprises six categories each for PLS aims and PLS characteristics. Twenty-five articles empirically investigated effects of PLS on several outcomes, but study designs were too heterogeneous to identify definite criteria for high quality PLS. Insular studies identified effects on accessibility and knowledge outcomes. For most criteria we identified from PLS writing guidelines, we found no empirical evidence. Conclusion and Implications: Considerable work to establish and investigate PLS has been done, but empirical evidence on criteria for high-quality PLS is still scarce. The conceptual framework developed in this review may provide a valuable starting point for future guideline developers and PLS researchers.