7. Conclusions
Due to the growing interest in the research and application of PPP procurement, this study presented a detailed review of the status quo, gaps, and research suggestion for the future of PPP research for infrastructure projects. The main contributions of this study are fourfold. First, the overview of PPP researches is based on the title of the study, publication year, journal title, country or region where the studies are conducted, project types, disciplines of the screened studies, and selection of the research articles. Theoretical foundations of PPP research in disciplines is set up to analyse the main theoretical foundations and special theories. Second, unlike most preceding literature reviews, this review is guided by a systematic process. It combines a three-phase word frequency analysis, cluster analysis, and an assessment of potential research topics. This process helps to summarise the appropriate topics related to PPP and to overcome some of the arbitrariness and subjectivity involved in research topic analysis. The word frequency and cluster analyses, which have a higher degree of similarity on the basis of the occurrence and frequency of words, are clustered and used to explore the status quo and research topic more quantitatively. Third, six research topics are explored, including financial package and PPP application, economic viability and VFM, risk management and success factor, procurement and contract management, performance management, and governance and regulation. In addition, relationship between different research groups is analysed, based on the theoretical foundations framework. Fourth, a research gaps framework is set up, based on research topics and the theoretical foundations of PPP research. On this basis, several research gaps are identified and research directions are provided. The methods for conducting word frequency and cluster analyses together with the process of research gaps identification, based on the systematic theoretical analysis, reduce subjective judgment in current research effectively. The findings can provide some conclusions on the status quo of PPP research and guide researchers in selecting the research topic in the future.
