## 3 Results and Discussion
### Answering RQ1 and RQ2  
RQ1: What is the current state of AR-based applications in manufacturing? RQ2: How does AR-based quality control benefit manufacturing in the context of Industry 4.0? The distribution of AR-based solutions in the Maintenance, Assembly, Quality, Other and General manufacturing contexts are 19%, 38%, 16%, 6% and 21%, respectively, as depicted in Figure 7. In more detail, the number of AR articles in each application field con \begin{table}
\begin{tabular}{c c c} \hline \hline
**Article** & [30] & [188] \\ \hline Type of paper & Application paper & Application paper \\ \hline Application field & Quality & Quality \\ \hline \multirow{4}{*}{Objectives} & Supporting operator & \\  & Improving process & \\  & Preventing human errors & Reducing human mental workload to \\  & Reducing movements & reduce errors in performing a task \\  & between workstation and & \\  & display system & \\ \hline Layer 1 & n/a & n/a \\ \hline Layer 2 & Interaction design & Interaction design \\ \hline Layer 3 & Usability + Effectiveness & Usability + Effectiveness \\ \hline Layer 4 & Field experiment & Field experiment \\ \hline Layer 5 & n/a & n/a \\ \hline Display devices & Monitor/Large screen & Android tablets \\ \hline Tracking method & Markerless & Marker-based \\ \hline Development platform & OpenCV, PCL, ROS & ARCore framework and SDK, Unity \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Example of data extraction from selected papers for the SLR. sidered within this paper's objectives from the year 2010 to 2021 is illustrated in Figure 8, which provides a longitudinal viewpoint for analyzing patterns, themes and trends concerning the application field in the quantity of publication. The timeframe from 2010 to 2021 is extensive enough to determine the evolution of literature in each field. It is not surprising that assembly is the leading adopter with 75 articles, or 38% of the total. This demonstrates a sustained interest in AR-assisted assembly, which peaked in 2019. Undoubtedly, assembly is the dominant sector in manufacturing to embrace AR technology. This is due to the nature of manual and semi-manual assembly activities that required the intensive involvement of operators, whose work is visual-oriented and who are in need of visual aid supporting. Next, when it comes to AR-based industrial applications in a specific field, maintenance is the second dominant sector, with 38 articles, or 19% of the total. Although the amount of AR-based maintenance applications fluctuates over time, they get the consistent consideration in 3 consecutive years from 2017 to 2019. Despite the investigation of AR solutions for the quality sector, this area is still far behind with 32 articles, or 16% of the total. Recently, this area has significantly emerged, reaching its peak in 2019, catching up with the AR articles for maintenance sector. Other sectors consisting of AR-assisted robot programming [211, 214], machine tool setup [213], real-time manufacturing modeling and simulation [121] have slowly been considered. General  
Figure 8: Distribution of AR solutions in different fields over years. Figure 7: Distribution of application field in manufacturing. manufacturing context solutions are relevant to those articles investigating generic AR-based solutions that can be customized and adopted for any particular field later to support the objectives of that field [123, 221]. Virtual and real context fusion, as well as the intuitive display, is the main advantage of implementing AR-based solutions for maintenance and assembly instructions. Thus, media representation in the forms of text, symbols, indicators, 2D symbols, 3D models, etc. could be directly projected on the relevant objects [100, 104, 144, 147, 165]. A comparative study for AR-based assisted maintenance was conducted to compare maintenance efficiency in using different assisting tools such as video instructions, AR instructions and paper manuals.