## 3 Results
### Publication Trends  
Figure 2 reveals a small number of publications on AR-supported instruction in an educational context of professional training, with an overall upward trend during the past two decades. The results indicated that the application of AR in professional training did not gain much attention from researchers in the past; however, the prospects for the field were hopeful. As shown in Figure 2, there has been a significant increase since 2016, when Niantic and Nintendo launched Pokemon Go, a hugely popular location-based AR game. Within one week, the game had attracted over 65 million users and made the general public aware of this technological innovation [18]. As a result, the release of Pokemon  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Category** & **Code** & **Description** \\ \hline \multirow{8}{*}{Basic information} & Title & Full title of the study \\  & Authors & Complete list of author names \\  & Year & Publication year \\  & Source & Information about the journal/book/URL \\  & Research type & Empirical/theoretical/synthesis \\  & Disciplines & Engineering/health and medicine/other \\  & Implementation setting & Formal/informal \\  & Input & Voice/magnet/motion/haptic/GPS/mouse and keyboard/scanner/other \\  & Output & Monitor/video \\ Technology features & Computing devices & Desktop/laptop/mobile device/wearable device \\  & Media representation & Symbol/indicator/text/data/2D image/3D object/video/animation \\  & Interactivity & High level/low level/no interaction \\  & Instructional function & Attention grabber/content delivery/practice/assessment/engagement/other \\  & Pedagogy & Game-based learning/trial-and-error/direct direction/experiential learning \\  & Scaffolding & No scaffolding/manual/computer \\  & Learning outcomes & Knowledge/behavior/skill/affective \\  & Data source & Content Tests/surveys/interviews/videos/fieldnotes/other \\  & Statistical & Difference (\(\ell\)-test/ANOVA/MANOVA/ANCOVA/non-parametric), \\  & Results & & \\  & & meta-analysis \\  & Effect size & Record if mentioned \\ \hline \hline \end{tabular}
\end{table}
Table 1: Lists of codes for the analysis of selected articles.  
Go is believed to have promoted the commercialization and popularity of AR products. The maturity of AR technology and the media hype about it has led to its increased adoption and experimentation in the educational context. Based on AR development, we compared studies in two time periods: one from 2001 to 2015 and another from 2016 to 2020. Divided by the launching year of Pokemon Go, the two periods represent two phases of AR development and comprise relatively equal publication records.  
Four categories of research studies were published from 2001 to 2020: theoretical research, empirical research, synthesis, and design case. As shown in Figure 3, the trend of publications in each research type is represented by colored lines. In this line chart, empirical research and design cases were the most common research types, but their evolving trends were different. The number of design cases increased before 2010 in general, reached the peak in 2010, and sharply decreased to zero in 2015. Regarding empirical research, there existed an overall rising trend, with a more significant increase occurring after 2011. Consequently, empirical research has been the mainstream research since 2011 to investigate AR usage in professional training. Compared with the two research types, theoretical research and synthesis were quite rare. Only one theoretical research was found in 2018, which discussed the impacts and application of AR on training in the age of e-learning [19]. Moreover, two synthesizing studies on AR in professional education were both published in 2020, respectively, reviewing applications of AR in organ transplantation [20] and research on AR used in sports education and training [21].