A review to identify key perspectives in PBL meta-analyses and reviews: trends, gaps and future research directions.

In the past 50 years, the original McMaster PBL model has been implemented, experimented, revised, and modified, and is still evolving. Yet, the development of PBL is not a series of success stories, but rather a journey of experiments, failures and lessons learned. In this paper, we analyzed the meta-analyses and systematic reviews on PBL from 1992 to present as they provide a focused lens on the PBL research in the past 5 decades. We identified three major waves in the PBL research development, analyzed their impact on PBL research and practice, and offered suggestions of research gaps and future directions for the field. The first wave of PBL research (polarization: 1990-mid 2000) focused on answering the question "Does PBL work?" and the outcomes. The results were conflicting. The researchers took polarizing positions and debated over the merits of PBL throughout this wave. However, the contradictory results and the debates in fact pushed the researchers to look harder for new directions to solve the puzzle. These efforts resulted in the second wave (from outcomes to process: mid 2000-mid 2010) that focused on the question "How does PBL work?" The second wave of PBL research targeted at investigating the effects of implementation constituents, such as assessment formats or single versus curriculum wide implementations. The third wave (specialization: mid 2010 and onward) of PBL research focused on "How does PBL work in different specific contexts?" These research widened our perspectives by expanding our understanding of how PBL manifests itself in different contexts. Given the diversification of PBL and more hybrid PBL models, we suggest "Why does PBL with particular implementation characteristics for specific outcomes work or not work in the condition where it is implemented?" to be the question to answer in the next wave of PBL research.