## 5 Discussion
### Methodological Concerns  
Concept papers have flourished in the Industry 4.0 for construction research topic. This review provided a summary of the current state of the research related to Industry 4.0 in the construction industry, both conceptually and providing an in-depth systematic discussion. From 20 targeted papers, 9 papers were concept papers. Concept papers generally contain a clear description of the research topic, including a summary of what is already known about that topic and the importance of the studies without documenting statistical evidence of the sources for the purpose of attracting readers to understand what the researcher is currently investigating, usually published during earlier stages of research [76]. Four papers were categorized as systematic literature reviews (SLRs), reviewing the trend toward digitization and automation of the construction industry and identifying and classifying the pattern of the research themes. SLRs are used to comprehensively locate and synthesize related research, using organized, transparent, and replicable procedures at each step in the process [77]. Another method of review used in the summarized papers was Longitudinal Literature Review (LLR). LLR is the science of tracing changes by repeatedly measuring the same phenomenon under the same circumstances over a long period of time [78]. This procedure is often used in standard SLR, but is differentiated by the extensive length of the observation period to identify the changing trend and determine how the trend influences the surroundings. This targeted review of these specific types of papers is able to help other researchers to overview what is currently happening to the construction industry due to the dramatic change due to the Industry 4.0 era. The suggestions for future research and the overview of the benefits and challenges of the research topic were rigorously documented using the evidence from the implementation in other industries.  
Original research (primary sources) of the targeted papers was limited. Only seven original papers were found, all of which used quantitative research on specific research topics that were not related to each other: augmented reality [58], facility management [53], structural monitoring [43], data management [48], design management [52], and real-time project management [49]. All selected papers discussed experiments and the result obtained. The articles included detailed descriptions of the methods used to produce the results for future verification or knowledge transfer by other researchers.  
To obtain an in-depth understanding of the theories underpinning Industry 4.0 for construction, qualitative research is needed as it focuses on \"why\" rather than the \"what\" to examine the natural phenomena of the research topic. However, the selected papers contained no qualitative original research, as the research topic is still in its infancy. Qualitative research papers are able to dive deeper into the problem of the research topic by reporting the phenomena from multiple perspectives, identifying many factors involved in the situation, and generally sketch the bigger picture that emerges. This limitation provides an opportunity to further explore this research topic qualitatively as the phenomena is immature due to a lack of theory and a limited number of available studies.