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.

中文翻译：

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确定 PBL 元分析和评论中的关键观点的评论：趋势、差距和未来的研究方向

在过去的 50 年中，最初的 McMaster PBL 模型已经实施、试验、修订和修改，并且仍在不断发展。然而，PBL 的发展并不是一系列的成功故事，而是一段实验、失败和经验教训的旅程。在本文中，我们分析了 1992 年至今关于 PBL 的荟萃分析和系统评价，因为它们提供了过去 50 年 PBL 研究的重点镜头。我们确定了 PBL 研究发展中的三大浪潮，分析了它们对 PBL 研究和实践的影响，并就该领域的研究空白和未来方向提出了建议。第一波 PBL 研究（极化：1990-2000 年中期）专注于回答“PBL 有效吗？”这个问题。和结果。结果相互矛盾。研究人员采取了两极分化的立场，并在整个浪潮中就 PBL 的优点进行了辩论。然而，矛盾的结果和争论实际上促使研究人员更加努力地寻找新的方向来解决这个难题。这些努力导致了第二波（从结果到过程：2000 年中期至 2010 年中期），重点关注“PBL 如何运作？”这个问题。第二波 PBL 研究旨在调查实施要素的影响，例如评估格式或单一与课程范围实施。PBL 研究的第三波（专业化：2010 年中期及以后）侧重于“PBL 如何在不同的特定环境中发挥作用？” 这些研究扩展了我们对 PBL 如何在不同环境中表现出来的理解，从而拓宽了我们的视野。鉴于 PBL 的多样化和更混合的 PBL 模型，我们建议“为什么具有特定实施特征的特定结果的 PBL 在其实施的条件下有效或无效？” 成为下一波 PBL 研究需要回答的问题。