## 2 Literature Review
### Research Gap  
Lean management strives to optimize production processes by identifying added value and eliminating waste. At its core is the CI principle, which encourages employee participation, self-management, and decision-making, underscoring a strong focus on employees. A KPI in lean management that identifies all types of waste is the OEE. Since 2019, research has been intensively discussing the integration of lean management and digitalization, although the exact interplay between the two is not yet fully understood. I4.0 is strongly technology-oriented, while lean management focuses on processes, people, and CI. Two perspectives can be distinguished here. Lean management is seen as the foundation for I4.0, as processes must first be optimized and standardized before new technologies can be implemented. The other perspective recognizes the limitations of lean management in specific contexts and views I4.0 as an opportunity to overcome them. Linking CI as a core principle of lean management and I4.0 is a promising approach to increase efficiency. It enables more effective communication, as I4.0 technologies can improve the interaction between people, machines, and products. It also promotes real-time decision-making through improved data collection, transparency, analysis, and visualization. In addition, lean management and I4.0 increase flexibility and productivity in production by minimizing waste and automating processes. They improve quality by allowing for better control over the production process and identifying defects early on. Finally, the integration of lean management and I4.0 can help reduce inventories and increase the reliability of production processes, leading to an overall increase in performance. The research up to now has not highlighted the influence of digitalization and CI.  
Theoretical research is characterizing the link between and influence of lean management and I4.0. There is a need for empirical research on how lean management and I4.0 can be effectively combined and implemented in practice. We have also seen in Table 1 that topics such as \"Flexibility\", \"Productivity\", and \"Reduced inventories\" are already being investigated to a greater extent. On the other hand, \"Quality\", \"Reliability\", and \"Real-Time Decision-Making\", as well as \"More efficient communication\", are still little-investigated areas. In the case of the combination of lean tools and I4.0 technology, we have also showed that \"Kanban\" and \"Poka Yoka\", for example, have been studied a lot, whereas Kaizen, among others, has received little to no recognition.  
We suggest that the Plan-Do-Check-Act (PDCA) cycle provides a good framework for continuous improvement (CI) through Industry 4.0 (I4.0) technologies. PDCA is a key method for CI. We hypothesize that in the Plan phase, real-time data analytics, digital twins, and IoT sensors could provide accurate insights for informed decision-making. In the Do phase, automation, MESs, and digital work instructions could be used to ensure accuracy and consistency. In the Check phase, more accurate and detailed analytics and real-time dashboards could enable continuous performance evaluation. In the Act phase, digital documentation and cloud-based knowledge management could standardize improvements and promote iterative learning. By embedding digitalization into each PDCA phase, we suspect companies will increase their transparency, accuracy, and adaptability, ensuring sustainable process optimization.  
The literature review outlined above emphasizes that more practical research is needed to show how specific lean principles and tools are impacted by digitalization, as this has not been adequately researched. CI is one such lean principle for research. An additional significant gap in the research is the availability of use case studies.