# Is Digital Twin Technology Supporting Safety Management? A Bibliometric and Systematic Review
## 4 Results  
### The Digital Twin Research Field  
Conference papers represent essential documents for supporting the scientific development of new research and application fields. As shown in Figure 4, the analysis of the dataset according to the type of document shows that the majority are conference papers (57%), followed by journal articles (33%), conference reviews (4%), and book chapters (3%). These results represent important evidence that allows Digital Twin research to be identified as an emerging research field. Figure 5 illustrates the progression of publications available in the Scopus data on Digital Twins in the period 2003-2021. There was an upsurge in publications from just 25 documents published in 2003 to 1279 documents in 2020. Research on Digital Twins saw a sudden spurt from 2017, which could represent a point of discontinuity, with an exponential shape in the last three years (2018-2020). The main reason for this surge may mainly be attributed to the governmental ICT (Information and Communication Technologies) investments in many countries carried out by the adoption of Industry 4.0 policy plans [43]. Even if there are more conference papers than journal articles in absolute terms, in the last three years (2018-2020), the share of journal articles over the total published documents regarding DTs has increased noticeably, going from 24% to 35%, registering an opposite trend compared to conference papers, which fell from 67% to 52%. Table 1 lists the top countries affiliated with authors of DT research, with the leading three being Germany (492 documents), the United States (477 documents), and China (434 documents). They are the top three global manufacturing export countries in the world [44], and Germany, in addition to having a leading position in Europe (Figure 6), is the country where the Industry 4.0 concept was first developed [44,45]. A noticeable self-perpetuating effect of giving and taking references was observed in these countries, which register more study contributions to the pool of the DT field. Countries that registered a high production were also among the countries whose studies received more citations. It can be said that the most well-established countries are dominating or leading in the field, and this can also be seen in the citation patterns. Figure 4: Publications in the DT research field by document type. Figure 5: Annual publication trend of 3301 documents retrieved from Scopus for the DT research field in the period 2003–2021. \begin{table}
\begin{tabular}{c c} \hline \hline
**Country** & **Number of Documents** \\ \hline Germany & 492 \\ United States & 477 \\ China & 434 \\ United Kingdom & 241 \\ Russian Federation & 205 \\ Italy & 164 \\ France & 144 \\ India & 116 \\ Spain & 95 \\ South Korea & 89 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Top ten affiliated countries publishing on DTs. Figure 6: Map of affiliated countries publishing on DTs. Keyword analysis was performed to explore the most prevalent themes in the DT research field. A total of 15,901 keywords were identified in 3301 documents. It is important to evaluate the keywords of a document to understand how authors frame their work, what the most interesting aspect of the research is or how it is evolving, and what trends are being created. From the extracted document sample, a count of the index keywords was performed in order to calculate their frequency and rank them. Table 2 shows the ranking of the top ten most relevant Keywords-Plus (ID). The ID is standardized; it is defined by Scopus to help in the research of documents associated with a topic. \"Digital twin\" is the most frequently used keyword, with 1404 occurrences, which indicates that this word alone is used as a termed concept in the literature. The other most frequently used keywords are \"life cycle\" (333 occurrences), \"manufacture\" (325 occurrences), \"embedded systems\" (248 occurrences), \"Internet of Things\" (206 occurrences), and \"Industry 4.0\" (200 occurrences).