3.5.1. Analysis of Highly Cited Literature

Usually, scholars cite the research results of their predecessors in their papers and list them in the form of references. Mutual citations of scientific literature reflect the objective laws of scientific development [38]. In Figure 7, the early citation network is relatively sparse, and the middle and late citation networks are denser. At the same time, it can be seen from the location of some large nodes that highly cited documents also appear in the middle and late periods.

**Figure 7.** Knowledge map of co-citation literature. The selection criteria for the study of literature co-citation networks is top 50 per slice, and the largest citation sub-network is displayed.

With reference to Figure 7, this study selected the top ten documents cited by frequency and detailed information of these documents listed in Table 6. It should note that the citation frequency in this article is limited to the mutual citation between these 14,692 articles so that the specific citation frequency will be different from the statistics in Web of Science. The article "Towards a common definition of global health" published by Koplan J.P. is cited most frequently [2]. About the term "global health", the academic community has not unified its final definition. At present, the explanation given by Koplan J.P. scholars recognized by the academic community. The burst value in the last column of the table indicates that the literature has received significant attention for a certain time. The highest burst value is the article "Global and regional mortality from 235 causes of death for 20 age groups in 1990 and 2010" published by Lozano R. et al. [39]. The rest of the literature is mainly on global health disease burden research, global health status survey, medical education, and global surgical exploration. Eight of the top ten most frequently cited articles are from the Lancet journal.


**Table 6.** Top 10 cited documents.

3.5.2. Cluster Analysis of Literature Co-citation Network

As an exploratory data mining technology, cluster analysis used to analyze and determine important topics, content, and evolution trends. Cluster analysis of literature co-citation can effectively classify a large number of similar research documents into a single knowledge unit, and then objectively reflect the main content of each knowledge unit [47]. The literature clustering knowledge map is shown in Figure 8.

**Figure 8.** Knowledge map of literature cluster. In CiteSpace, clusters are named by citing the keywords of the literature, and the log-likelihood algorithm (LLR) used.

In Figure 8, the color of the cluster blocks from cold to warm represents the average time of clustering from far to near. The red nodes in the cluster color block represent the literature with burst value. The more red nodes in the cluster block, it shows that this clustering topic is the research frontier and hot spot. In order to further understand the clustering theme, we have summarized the detailed information of clustering and plotted it as Table 7.


**Table 7.** Summary table of cluster information.

Note: The silhouette value is the parameter used by CiteSpace software to evaluate the clustering effect. Specifically, the evaluation of clustering measuring the homogeneity of the network. The closer the silhouette value is to 1, the higher the homogeneity of the network and the clustering results with high reliability are greater than 0.7.

It can be seen from Table 7 that the silhouette values of all clustering results are greater than 0.7, indicating that there is no problem with clustering. According to Figure 8 and Table 6, currently, "global

health governance", "global health diplomacy", "medical education", "global health education", "antimicrobial resistance" and "Zika Virus" are the research frontiers and hot spots in the global health field. In addition, "quality of life", "energy balance; adipose tissue; appetite", "subunit vaccine", "mental illness", "mental health", "community impact", "health services" and "interaction strategies" are important research topics and directions of global health.
