## 4 Methodology
### Systematic Literature Review Process  
Data was retrieved in January 2022. With a view to covering all the literature around this specific topic throughout all the previous years, no year limitation was set. A pertinent and thorough search equation was used to report the literature on the state-of-the-art while addressing all educational stages and topics. Consequently, and due to the interdisciplinary nature of the topic, the following query using wildcards and logical operators was used: \"('augmented reality') AND ('gamif*') AND ('education' OR 'universit*' OR 'college*' OR'school*' OR 'butent*' OR 'pupil*' OR 'teach*' OR 'learn*')\". In SCOPUS, WoS, IEEE, and ERIC databases, the search involved the title, abstract, and keyword parameters, while in Google Scholar the \"allintite\" operator was used along with the keywords in consecutive order (e.g., 'augmented reality' AND 'gamification' AND 'education'; 'augmented reality' AND 'gamification' AND 'university', etc.).  
The whole process, which is displayed in Figure 1, followed and abided by all the steps and guidelines of the PRISMA statement. Initially, 670 documents were reported in the 5 databases (314 in SCOPUS, 204 in WoS, 80 in IEEE, 53 in Google Scholar, and 19 in ERIC). Of these documents, 220 were duplicates and were not included. Hence, 450 documents were screened. The main inclusion criteria were the combinational use of augmented reality and gamification elements, the reference to the educational context, and the studies involving either an empirical study, the development of an educational application, a proposal or prototype, a systematic review, or theoretical contributions. In total, 316 documents did not meet the research criteria and were excluded from the study. All of the 134 documents that were sought for retrieval were successfully retrieved. Therefore, 134 documents were examined for eligibility. In addition, 21 studies were excluded as they did not meet the necessary research criteria. Consequently, 113 studies were included and analyzed in the review.  
The 113 studies identified were divided into three categories; that is, (1) empirical studies (73 articles, pct. = 64.6%), (2) proposal and prototype papers (design-oriented without being applied in educational contexts) (27 articles, pct. = 23.9%), and (3) review, conceptual, and theoretical papers (13 articles, pct. = 11.5%) (RQ2). The review, conceptual, and theoretical papers were scrutinized and their main findings were identified. Regarding the proposal and prototype articles, suggestions, guidelines, practices, areas of focus, and findings were also examined and analyzed. The empirical studies were analyzed and compared according to the following variables:  
1. Country in which the experiments were conducted;
2. Educational stage;
3. Focus area;
4. Developmental category;
5. Sample;
6. Main aims;
7. Research method;
8. Main variables;
9. Measurement--research instruments and tools;
10. Application name;
11. Application development methodology;
12. Development tools;
13. Operating system;
14. Devices used in the experiment;
15. Gamification elements;
16. Main findings.  
Figure 1: Prisma flow diagram.