## 2 Research Methodology
### Data Extraction and Analysis  
#### 2.3.1 Classification Framework  
The classification framework used to analyze AR-based publications in manufacturing and extracting relevant information for answering RQs consists of three parts:  
1. Application area in manufacturing mixed categories of papers:  
At the beginning of the 200 selected publications, each can be allocated into five solution groups according to their application field in an industry 4.0 context: (1) Maintenance, (2) Assembly, (3) Quality, (4) Others and (5) General manufacturing context. Next, they were classified into 4 different categories of papers following the benchmark in [32]: review papers, technical papers, conceptual papers and application papers (see Table 5). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Paper type/** & **Maintenance** & **Assembly** & **Quality** & **Others** & **General Manufacturing** \\ \cline{2-6}  & 2 articles & 3 articles & 0 & 0 & 16 articles \\ \cline{2-6}  & [33,34] & [35, 36, 37] & & & [2,17, 18, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50] \\ \hline Technical & 1 article & 18 & 2 articles & 0 & 5 articles \\ \cline{2-6}  & [51] & [52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 66, 67, 68, 69] & [70, 71] & [72, 73, 74, 75, 76] \\ \hline Conceptual & 15 articles & 22 articles & 5 articles & 3 articles & 16 articles \\ \cline{2-6}  & [77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91] & [113, 114, 115, 116, 117, 118] & [119, 120, 121, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137] \\ \hline Application &The correlation between these two classifications formed a matrix giving an overview of the current interest in AR-based solutions in the industry. In more detail, review papers are the ones summing up the current literature on a specific topic to provide the state of the art of that area. Technical papers are mainly about solutions and algorithms for the development of hardware/software and AR systems. Conceptual papers consider specific characteristics of AR solutions to propose advanced concepts for their further practical adoption. Finally, application papers provide works that develop and test AR solutions in a case study or real environment. With this classifying approach, the results in Figure 5 show that there is currently no systematic review paper about AR technology enhancing quality sector. The general manufacturing context has the highest number of review papers, while Assembly has the highest number of application papers. Although the total number of publications of AR-based quality is less than the total number of articles in AR-based maintenance, AR-based application papers in quality are slightly higher than in maintenance. Considering that the investigation for AR-based quality solution was behind maintenance in the past, this proves that the interest for implementing AR technology in quality sector has significantly grown in recent years. 2. The architecture layer framework of AR systems in manufacturing  
After the first classification, each paper's content was analyzed following the architecture layer framework of the AR system adapted from [226], as in Figure 6, to extract relevant data for answering the RQs. This architecture layer framework of AR systems was adapted and improved from a study in the built environment sector. The framework was chosen for the analysis step because its architecture was constructed in accordance with the standard architecture layer criteria for developing information technology concepts and tools. Besides that, it could cover all essential aspects of an AR application based on system point of view (layer 1, 2), industry point of view (layer 4, 5) and user point of view (layer 3: usability, layer 2: interaction design, content design).