When systematic review meets realist review: proposing a hybrid review design to understand complex intervention...

Systematic reviews (SRs) and realist reviews (RRs) are increasingly conducted in a coordinated manner with intersecting methodological processes, forming what we term hybrid review designs. However, little is known about how these hybrid reviews are conducted and structured.

This study aimed to characterize existing hybrid reviews by examining their features, analyzing their method intersection strategies, and synthesizing the reported advantages and challenges, thereby informing future methodological guidance.

A scoping review following the Joanna Briggs Institute approach was conducted. We searched the following databases from January 2005 to May 2025: PubMed, Web of Science (Clarivate), Embase (Ovid), PsycINFO (Ovid), and CINAHL (EBSCOhost), supplemented by citation searches. Eligible studies were hybrid reviews that explicitly integrated SRs and RRs in a coordinated manner. Data on study characteristics and methodological features were analyzed descriptively, while intersection strategies and the reported advantages and challenges were synthesized using qualitative content analysis.

Of 129 included studies, we classified hybrid review designs into 4 types based on methodological features: SR and RR in separation (SR|RR, 48 of 129, 37.2%); SR and RR in combination (SR + RR, 15 of 129, 11.6%); realist-informed SR (47 of 129, 36.4%); and multiphase hybrid review (19 of 129, 14.7%). Within the SR|RR and SR + RR types, 8 method intersection strategies were identified, with the most common being "using SR to inform the initial program theory development of RR" in SR|RR (27 of 48, 56.3%) and "sharing search strategies" in SR + RR (11 of 15, 73.3%). Reported advantages included stronger evidence, actionable insights, aiding complex intervention/theory development, and streamlined processes; challenges included philosophical/methodological tensions and labor intensity.

This review provides a foundational map and classification of hybrid reviews. The findings illuminate how SR and RR methods are integrated in practice, establishing an evidence base to inform methodological guidance and supporting researchers in designing such reviews.

This study looked at how 2 types of research reviews-systematic reviews, which focus on whether interventions work, and realist reviews, which focus on how and why interventions work-can be conducted in a coordinated manner. We reviewed 129 published hybrid reviews and identified 4 types of integration, as well as 8 common method strategies for linking the 2 approaches. We also summarized the reported benefits and challenges of these hybrid reviews. Our findings show that hybrid reviews have the potential to provide a more comprehensive understanding of interventions by combining evidence of effectiveness with insights into underlying mechanisms. This information can help researchers plan future reviews that better inform policy and practice and guide journals and funders in setting reporting standards for these innovative review designs.