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8 February 2024

Public–Private Partnership (PPP) in Road Infrastructure Projects: A Review of Evolution, Approaches, and Prospects

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,
and
1
Department of Accounting and Finance, Faculty of Economics and Business Studies, University of Granada, 18071 Granada, Spain
2
Department of Accounting and Finance, Faculty of Social and Legal Sciences, University of Granada, 52005 Melilla, Spain
3
Department of Management-1, University of Granada, 18071 Granada, Spain
*
Author to whom correspondence should be addressed.
Sustainability2024, 16(4), 1430;https://doi.org/10.3390/su16041430
This article belongs to the Section Sustainable Engineering and Science
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Abstract

Public–private partnerships (PPPs) are a widely used procurement method for constructing, maintaining, and operating roads, constituting an alternative to the traditional public works model (TPW). This is because the efficiency of the PPP model has been highlighted over the TPW model. This study carried out a bibliometric analysis of 734 public articles from Scopus on PPPs in road infrastructure projects between 1993 and 2022. The results allow us to identify the general characteristics of the research; the most relevant articles; the most productive journals; and the most prolific authors, institutions, and countries and their main international cooperation networks, as well as to detect the main current research topics, which are: financial management, costs and pricing systems, privatization of transport infrastructure services, and sustainability. Consequently, this study provides a comprehensive overview of research on PPPs in road infrastructure projects over the last 30 years. This can reinforce and complement previous bibliometric analyses on PPPs in road infrastructure projects.

1. Introduction

Road infrastructure is an essential component of economic development and societal connectivity. Public–private partnerships (PPPs) are a promising approach to raising private finance for road construction and maintenance. PPPs are contracts that last for a long time between the public and private sectors to provide public services and infrastructure and include sectors like transportation, energy, environment, health, security, and education [1,2,3,4]. In many countries, PPPs are becoming an increasingly popular way of procuring infrastructure and public services, as they allow governments to secure infrastructure without raising taxes or going into immediate debt [5]. The public sector encourages the private sector to get involved in infrastructure projects in developing or developed countries. The objective is to reduce the gap between the growing demand for road infrastructure and budgetary constraints and the ineffectiveness and inefficiency of traditional public procurement to address this road infrastructure deficit [6,7]. The adoption of PPPs has attracted increasing research interest in recent decades. This has translated into a remarkable increase in the volume of articles published and the variety of topics addressed, fields of study, and methodological approaches employed.
There are multiple studies in the previous literature on PPPs in infrastructure projects [8,9,10,11,12,13,14,15,16,17], so some authors argue that they tend to lack a holistic and integrative vision [18], so this research carries out a global review of PPPs in road infrastructure projects over the last 30 years, the objective of this research being to analyze the evolution of the scientific production of PPPs in road infrastructure projects, to highlight the main advances during the period from 1993 to 2022. It highlights the most significant advances in research in this field during this period, identifies themes and trends of study, and points out the gaps in existing knowledge. Accordingly, the following research questions are posed:
  • Q1. What are the main characteristics of the research line?
  • Q2. What are the most influential publications?
  • Q3. Who are the most prolific contributors (journals, institutions, and countries)?
  • Q4. What is the trend in cooperation (journals, institutions, and countries)?
  • Q5. What are the main research topics and trends?
The present research is structured as follows: Section 2 presents the methodology, data source, and exclusion criteria of the analyzed documents, while Section 3 shows the main results that provide answers and evidence to the research questions posed (Q1 and Q2). Section 4 presents the top collaborators (Q3) and the trends in collaboration (Q4) across journals, institutions, and countries. Section 5 presents the main research topics and trends in PPPs in the road sector (Q5). In Section 6, you can find the main research findings, conclusions, limitations, and future lines of research.

2. Materials and Methods

2.1. Methodology Used for Analyzing Data

This study utilizes bibliometrics as a basis for its approach, which aims to identify, organize, retrieve, and evaluate research documents to examine the temporal evolution of a specific field [19,20,21]. Therefore, this methodology provides a comprehensive and systematic overview of the literature [22,23] to analyze the line of research, determining the fundamental issues, the most relevant and current methodologies, as well as the level of interest in the subject matter [24]. Consequently, in this study, we identify and analyze the fundamental elements of the interrelationship within the concepts of PPP and roads, presenting the metadata and trends found in diverse databases that address this specific field [21]. Additionally, network maps have been created and proposed using VOSviewer v.1.16.7 software to cluster and analyze related words.
In addition, we have created and proposed network maps using the VOSviewer program v.1.16.7 to organize and analyze related words. This software, being able to generate maps based on network data, facilitates the visualization and exploration of these maps. Additionally, VOSviewer can extrapolate and create networks encompassing publications, journals, countries, keywords, and research trends. Articles in these networks can be linked through co-authorship, co-occurrence, citations, bibliographic coupling, or co-citation links [25,26]. Van Eck and Waltman (2010) argue that the added value of this software lies in its ability to gather data from different scientific databases such as Web of Science, Scopus, Dimensions, and PubMed, as well as reference manager files like RIS EndNote and RefWorks [27].

2.2. Bibliometric Analysis Procedure

Three stages were used to apply the methodology, as depicted in Figure 1.
Figure 1. Methodology used.
Below is the methodological procedure used.

2.2.1. Identification Phase

We conducted an exhaustive review of the central databases, such as Google Scholar, PubMed, and Web of Science. Finally, we chose to use Elsevier’s Scopus as the search engine, adhering to the suggestions of Harzing and Alakangas (2016) and Mongeon and Paul-Hus (2016) [28,29]. The rationale is that (a) it is the scientific repository that provides the most information on authors, institutions, and countries [30]; (b) it has a lot of articles and journals that meet scientific quality requirements [31]; and (c) in comparison with the Web of Science, it offers a broader coverage [32]. Given its recognized suitability for bibliometric reviews, several authors have considered Scopus to be the most appropriate scientific database [33].
A total of 1287 research papers meeting the search criteria were found. However, only some of these documents were considered for the analysis; different inclusion/exclusion criteria were applied to complete the research objective. The first criterion consisted of the type of paper that is scientific, where only research articles were selected. This is because novelty and exhaustive peer review are valued, indicating higher scientific quality [34,35]. Of the 1287 papers analyzed, only 769 met this search criterion. In addition, a time horizon covering research articles published between 1993 and 2022 was established, and as a result, 734 research articles that fulfilled the search criteria were retrieved.
The data were extracted on 23 May 2023. The search string used is as follows: (TITLE-ABS-KEY(“public-private partnership” OR “public-private partnerships” OR “private finance initiative” OR “build operate transfer” OR “build own transfer” OR “transfer operate transfer”) AND (“Highway” OR “road”)) AND (LIMIT-TO (DOCTYPE, “ar”)) AND (EXCLUDE (PUBYEAR,2023) OR EXCLUDE (PUBYEAR,1992) OR EXCLUDE (PUBYEAR,1990) OR EXCLUDE (PUBYEAR,1979)).

2.2.2. Analysis and Visualization Phase

The data was downloaded and analyzed on 23 May 2023. From the sample that meets the search requirements, interactions between authors, countries, and institutions are examined, as well as the development of keywords. International cooperative networks are crucial in creating novel and high-impact research, as well as fostering the creation of synergies and the sharing of ideas [36,37]. This analysis is carried out using the co-citation method, i.e., as the frequency increases, the interrelationships between them also increase, resulting in a more excellent conceptual relationship.
The keyword analysis is based on the co-occurrence method, which identifies a conceptual and thematic structure. Thus, the results present an overview of the most prominent research areas related to road PPP. Table 1 shows the specific methods used to carry out the bibliometric analysis.
Table 1. Methods used in the bibliometric analysis.

2.2.3. Results Stage and Discussion

The results are presented in terms of authors, journals, subject areas, countries, affiliations, and international cooperation networks. Maps are also created based on the co-occurrence of keywords and co-citations, commonly utilized in bibliometric studies. These maps provide a meaningful visualization of relationships and trends within the study area, e.g., studies by Castelblanco et al. (2021) and Van Eck and Waltman (2010) [27,38].

3. The Main Characteristics of Scientific Production and the Most Influential Publications (Q1 and Q2)

The main features of scientific production in the research line on PPPs in road infrastructure projects from 1993 to 2022 are presented in this section. The next are the results related to the growing number of published articles; the number of authors, institutions, and countries published in the row of research; the total number of citations and the average number of citations received; and the number of journals in which the papers have been published, fitting the average number of authors for each published article without any filtering [39].
Callon et al. (1986) argue that the number of publications in a specific period can measure research activity [40]. The evaluation of indicators allows us to understand the quantitative evolution and research topic’s structure [41]. The total period of 30 years has been subdivided into six subperiods of 5 years each (Table 2).
Table 2. Main characteristics of scientific production.
Since the publication of the first research article entitled “Development of a freeway traffic management project through a public-private partnership”, which analyzed the progress of a highway traffic management project across PPPs [42], all the indicators analyzed show exponential growth as a whole over the 30 years of development of the line of research. The comparison between the first subperiod (1993–1997) and the last one (2018–2022) reveals an increase of 925% in research articles, more than 1633% in authors and institutions participating in the line of research, 737% in countries, 731% in journals, and 982% in citations received.
Figure 2 represents the five-year evolution of the publications carried out in the field of research. It appears that there are two clearly differentiated periods: (a) the first period, from 1993 to 2007, shows a linear increase in the number of publications, indicating that the research topic is beginning to generate interest in the research community. The duration of this period saw the publication of 112 articles, thus laying the theoretical and conceptual foundations of the research line. (b) In the second period, from 2008 to 2022, exponential growth is evidenced, with the publication of up to 662 research papers.
Figure 2. Number of papers published in each subperiod. Research trend.
During this second period, many authors, institutions, countries, and journals joined the line of research (Table 2). This growth could be explained by the global financial crisis of 2008 when the PPP market in the road sector experienced a slowdown. However, in Latin America, an immediate recovery was observed, unlike in the European Union, where the PPP market in the road sector took longer to recover [43]. This fact entailed a growing interest among researchers at the international level, so the global financial crisis posed a challenge in the research line to find more modern formulas to suit the new framework for action.
Table 3 shows the 24 thematic areas in which the 734 research articles on PPP in road infrastructure projects were classified. It is relevant to remember that the sum of the articles published by each research area is more significant than the total number of articles published since a paper may simultaneously belong to one or more research areas. The main research areas in which the topic has been posted are Engineering; Social Sciences; and Business, Management and Accounting, representing 69.92% of the total scientific production.
Table 3. Area of research.
It is interesting to analyze the evolution of each subject area over different periods: the first from 1993 to 2007 and the second from 2008 to 2022 (Figure 3). Notably, the trend has remained constant throughout both subperiods, with Engineering and Social Sciences remaining the main subject areas. However, Engineering has lost importance in favor of other areas such as Business, Management and Accounting and Economics, Econometrics and Finance. In particular, the area of Engineering received the most attention during the first period (n = 75; 41.44%). However, in the second period, this area experienced a decrease in attention (n = 309; 27.39%). Social Sciences showed its relevance in the first period (n = 43; 23.76%) and remained constant in the second period (n = 265; 23.49%). Business, Management and Accounting showed some interest in the first period (n = 23; 12.71%) but experienced exponential growth in the second (n = 200; 17.73%). Similarly, Economics, Econometrics and Finance were minimally represented in the first period (n = 2; 1.10%) but experienced substantial growth in the second (n = 89; 7.89%). Decision Sciences recorded (n = 8; 4.42%) in the first period and experienced significant growth in the second period (n = 60; 5.32%). Finally, Environmental Sciences went from (n = 8; 4.42%) in the first period to (n = 57; 5.05%) in the second period. In short, the six thematic areas mentioned in the first period represented 87.85% of the scientific production. In contrast, in the second period, they reached 86.88% of the scientific output, so despite the variations in the relative importance of each of them within the line of research, they are considered the most productive thematic areas in research on public–private partnerships in road infrastructure projects.
Figure 3. Evolution of the areas of research.
The analysis of the areas themes reveals that throughout the entire period under analysis, “Engineering” has been the main (29.31%), followed by “Social Sciences” (23.51%), “Business, Management and Accounting” (17.10%), and “Economics, Econometrics, and Finance” (6.95%), with these four thematic areas accounting for 76.87% of the total number of publications. During the first period, 1993–2007, the most studied areas were Engineering (41.44%); Social Sciences (23.76%); and Business, Management and Accounting (12.71%). However, it was observed that during the second period (2008–2022), the area of Engineering (27.39%) decreased its relevance. In contrast, “Social Sciences” maintained its significance in both periods, as did “Business, Management and Accounting” (17.73%), and “Economics, Econometrics and Finance” (7.89%) experienced an increase. In particular, “Economics, Econometrics and Finance” was the fourth area of knowledge that contributed most to the PPP road research area.
Table 4 presents the main feature of the most cited research articles in the knowledge area.
Table 4. Top 10 most cited research articles on PPPs in road infrastructure projects.
The main academic contributions are grouped around several axes. Xu et al. (2010) developed a model for evaluating risk for road PPP projects in China. They concluded that this model could help governments and private companies improve the management of this type of project [44]. Along the same lines, Abednego and Ogunlana (2006) examined the perception of appropriate allocation of risk in PPP toll road projects in Indonesia and concluded that good governance improves project performance by promoting proper risk allocation [45]. Similarly, Ng et al. (2012), through a questionnaire and interviews targeting PPP experts in Hong Kong, identified three critical elements for success in the early stages of PPP project implementation: the existence of sustainable demand, the alignment of the project with government objectives, and the presence of a robust private consortium [46].
Yang and Meng (2000) analyzed the selection and evaluation of a road project using the build–operate–transfer (BOT) scheme in equilibrium networks. They considered pricing options, road capacity, and the effects of the choice of investigating profit and welfare. Their findings are valuable because they allow private investors to intuitively understand profitability and benefits and identify the conditions that make BOT-type projects viable and profitable [47]. Thomas et al. (2006), using the Delphi method and the fuzzy fault tree, put forward a risk probability and impact assessment framework and concluded that delay in financial closure, traffic income, demand, and land procurement are the most critical risks in BOT road projects [48]. Similarly, Iyer and Sagheer (2010) identified 17 essential risks of highway projects under PPP in India using the interpretive structural modeling (ISM) method to analyze the hierarchical structure and interrelationship of risks. They found that financial closure risk, schedule risk, and cost overrun risk were dependent on other risks, while 14 risks were autonomous and had little impact on each other. Cost overruns and schedule risks were the most vulnerable [49].
Ashuri et al. (2012) determined the pricing of minimum revenue guarantee options on BOT projects using finance/decisional science theory. They found that the proposed model contributes to analyzing and understanding the financial risk of BOT projects [50].
Brandao and Saraiva (2008) used market data to estimate the stochastic parameters of the project, especially to evaluate a minimum traffic guarantee that grants the concessionaire a subsidy from the State if traffic is lower than expected [51]. On the other hand, Rathi (2006) compared community participation and participation by the private sector in waste management in Mumbai, India. Community participation costs USD 35 per ton due to waste separation, while PPP participation costs USD 41 per ton by focusing on waste processing [52]. Finally, Kivilä et al. (2017) analyzed the organization’s control techniques to ensure sustainable project management. They found that sustainable project management results from a combination of indicators, control mechanisms, and sustainable project administration [53].

4. Partners and International Cooperation Networks (Q3 and Q4)

Below is a presentation of productivity results for authors, institutions, countries, and journals and their international cooperation networks. The 10 most productive authors in this research area are shown in Table 5.
Table 5. Top 10 most productive authors.
It is worth mentioning that 40% of the authors are of Asian origin, followed by 30% from the USA. Likewise, 20% of the authors are from Europe, while only 10% are of Latin American origin. However, of the top 10 most productive authors, only the American Garvin, M.J. and the Colombian Guevara, J. worked together to write two co-authored research articles.
The Spanish author Vassallo, J.M. stands out as the most prolific author, with nine published research articles. He is followed by Guevara, J.; Wibowo, A.; and Yang, H. with eight published research articles. Chan, A.P.C. obtained the highest average number of citations per article, with an average of 89.80, followed by Yang, H., with an average of 62.38 citations per article, who also obtained the highest total citations (449).
On the other hand, the analysis through VOSviewer for international cooperation networks reveals that there is no international network in this line of research. Instead, co-authorships are mainly between researchers of the same nationality, indicating that this line of research is being addressed exclusively by authors from the same country (see Figure 4).
Figure 4. Authors’ international cooperation networks on PPPs in road infrastructure projects.
Table 6 displays the top 10 institutions and their leading bibliometric indicators. The high concentration of institutions of Asian origin is noteworthy, representing 50% of the total, followed by institutions of North American and European heritage, each with 20% representation, and finally, institutions of Latin American origin with 10%. This broad international participation in the line of research is also evidenced by the fact that most of these institutions began publishing on the subject during the period of origin of the study (1993–2007), and all of them have continued to publish during the current period (2018–2022), indicating a high degree of interest in the scientific community. However, it is worth mentioning that international cooperation indexes have yet to be found.
Table 6. Top 10 most productive institutions.
The Universidad Politécnica de Madrid has the highest productivity, with 18 published research articles, although its H-index is the fifth highest (7). The Universidade de Lisboa follows it with 15 articles and an H-index of 3. The Hong Kong University of Science and Technology, the Hong Kong Polytechnic University, and the Universidad de Los Andes stand out with 10 research articles and H-indexes of 10, 8, and 6, respectively. The Hong Kong University of Science and Technology has obtained the highest dissemination of its research results, with 612 total citations and an average of 61.20. This gives it the highest standard among the top 10 most productive institutions. In second place is Hong Kong Polytechnic University, with 519 total citations and a mean of 51.90.
Figure 5 illustrates the international cooperation network of the institutions. An interplay of at least five research articles was selected, and 1256 institutions were found in total, of which only 56 were published in international cooperation networks on road PPPs, so it can be stated that collaborations are infrequent.
Figure 5. International cooperation networks of institutions on PPPs in road infrastructure projects.
Table 7 displays the 10 most productive countries. The distribution of the countries in road PPP research is more diverse than that of authors and institutions. A total of 40% of the countries are of European origin, while 30% are of Asian origin. The remaining 20% is divided between North America and Oceania.
Table 7. Top 10 most productive countries.
The United States is the country that publishes the most research articles in this field (187) and has the most total citations (4693), followed by China (96 articles). However, Hong Kong has the highest index (17), followed by Spain, which has an H index of 7. The United States stands out as the country with the highest volume of articles published in this line of research 187 in total. China ranks second in the number of articles published, with 96. Regarding impact indexes, Hong Kong ranks first with an H index of 17, while Spain ranks second with an H index of 7. On the other hand, Australia has the highest average number of citations (52.85), while Hong Kong (52.42) and the United Kingdom (44.76) follow closely behind.
Finally, there is a low trend toward international cooperation in general, with only Hong Kong (66.7%), Australia (61.7%), and the United Kingdom (51.6%) showing cooperation rates above 50%. Apart from these three countries and China (43.8%), there are limited numbers of international collaborators in each country.
Figure 6 illustrates the countries’ international cooperation network. An interaction of at least five published documents was selected, obtaining 41 countries grouped into eight clusters. As shown in Table 7, the United States, China, and the United Kingdom are the countries with the highest number of publications, and, as they are in the center of the map, they are the countries around which the different international cooperation clusters revolve.
Figure 6. International cooperation network of countries international cooperation networks of institutions on PPPs in road infrastructure projects.
Table 8 represents the most productive journals in road PPP research. The primary metrics analyzed are the total number of articles published, the total and average number of citations received, the Hirsch index in the research line on road PPP of the journal in SJR Scimago, the publisher, and the number of research articles in each period of the research line (Figure 2).
Table 8. Top 10 most productive journals.
A total of 50% of these journals are of North American origin, while the other 50% come from Europe. Of the 341 journals that published research articles (Table 2), those in Table 7 captured 178, 24% of the total scientific production disseminated through this medium. This indicates that, although there is a high number of journals, these are the references in the line of research, which is also supported by the number of citations received, the high average number of citations per research article published, the high Hirsch indexes, and the very high rating of their different research areas in the SJR Scimago. Notably, 10 journals started publishing during the exponential growth phase, while the other seven did so practically in the emergence phase. The most cited article in the research pipeline is “Tolling heavy goods vehicles” [54], published in Transportation Research Record.

6. Discussion and Conclusions

This study aims to analyze the research conducted on PPPs in road infrastructure projects globally between 1993 and 2022 through the bibliometric analysis of 734 research articles registered in the Scopus database. The number of published articles, authors, and co-authors, citations, average number of citations per article, and countries and journals of publication were investigated, showing that there has been a general increase in research interest in PPPs in road infrastructure projects, especially in the period 2008–2022, possibly driven by the global financial crisis of 2008.
There are numerous studies in the literature on public–private partnerships (PPPs) in infrastructure projects; for example, Tallaki and Bracci (2021) reviewed risks and their management [86]. Ahmed and Garvin (2022) analyzed critical success factors and key performance indicators of PPPs [87]. Pu et al. (2021) presented the evolution and trends of PPP research in China [15]. This study distinguishes previous studies, given that it presents the main authors, countries, institutions, and international cooperation networks, as well as the main research topics and future lines of research. In addition, this study provides a global review of PPPs in road infrastructure projects over the last 30 years.
Our findings show that the authors with the most products are Vassallo, J.M.; Guevara, J.; and Wibowo, A., with nine, eight, and eight published research articles, respectively. It is worth noting that 40% of the authors are of Asian origin, which could be because Asian governments have promoted, in recent years, the development of PPP projects for the provision of infrastructure and public services, which has generated a strong interest in the part of the international academic community in their study. Among institutions, the Polytechnic University of Madrid (Spain) was the most productive institution in terms of publishing research articles on PPPs on roads (18). This could be because this university has had a line of research focused on the economics and financing of transport infrastructure for over thirty years. It analyzes regulation, bidding, economic impact, pricing systems, and, in general, road planning and management. The enormous interest aroused by the scientific community in this line of research is because Spain has been a pioneer in the development of concessions for the construction and operation of toll roads since its first concession project dates back to the end of the 1960s. The experience in this sector has been exported to other countries worldwide. In turn, our findings identified that the United States generated the most significant number of research articles on highway PPPs (187). In addition, the United States (187), China (96), United Kingdom (64), India (59), Australia (47), Spain (35), Canada (28), Hong Kong (24), Portugal (20), and the Netherlands (14) are the countries that have published the most during the entire study period. However, it should be noted that 40% of the countries that do the most research in this line of research are of European origin, which could be because, in this area, the European Commission has encouraged the use of this type of financing for infrastructure projects and this has generated interest in many researchers from European institutions.
The analysis of the thematic areas of research reveals that, throughout the entire period that was analyzed, “Engineering” has been the primary area (29.31%), followed by “Social Sciences” (23.51%), “Business, Management and Accounting” (17.10%), and “Economics, Econometrics and Finance” (6.95%), with these four thematic areas accounting for 76.87% of the publications. It is worth noting that the area of “Economics, Econometrics and Finance” has been gaining considerable ground in recent years, which could be explained by the financial crisis at the international level and the growing need to address instruments to facilitate PPP and thus make investment in road infrastructure more sustainable. This means the main issues discussed in road PPP are related to financial management, costs, pricing systems, privatization of transport infrastructure services, and sustainability. Financial management is currently the most relevant line for the scientific community. At the same time, sustainability, which promotes sustainable technologies in road construction and maintenance, will capture research attention shortly.
Consequently, it is concluded that after the global financial crisis of 2008, the PPP market in the road sector experienced a slowdown. Meanwhile, in Latin America, the recovery was rapid, unlike the EU, where the PPP market took longer to recover [43]. This phenomenon has awakened the academic community’s interest in investigating this line of research, leading to a continuous increase in articles, authors, countries, citations, and scientific journals in the last decades. The Sustainable Development Goals (SDGs) can inspire researchers and practitioners to expand their points of view to address the implications of PPPs on roads. In addition, it is crucial to highlight the importance of the 17 SDGs, emphasizing the need to foster international collaboration to achieve sustainable development. This bibliometric analysis makes it possible to understand and assess the suitability of different actors, promoting and sharing knowledge and experiences, providing clues, and creating connections between relevant clusters and strands for the future.
Finally, this study has some limitations that provide the basis for the development of future research to complement the results obtained. One of the limitations of this work is that the results are not generalizable due to the specific characteristics of each project and the countries’ social, political, and economic conditions. Therefore, we recommend that in the future, the results collected here be compared with the specific characteristics of the specific case. On the other hand, using other databases, such as the Web of Science or Google Scholar, or broadening the type of research documents to include book chapters or papers presented at conferences, could expand the number of search documents and identify other relevant topics. At the same time, this would also be possible by broadening the search terms to avoid excluding records of scientific value, allowing for greater completeness of the analysis. The VOSviewer software tool was used to visualize the cluster data, which provides results that may differ from those obtained using other software. Lastly, a research design based on a more exhaustive content analysis would allow us to investigate the identified research topics more deeply.

Author Contributions

Conceptualization, Y.Y.E.-N.-d.-G., M.d.l.Á.B.-M. and V.M.-M.; methodology, Y.Y.E.-N.-d.-G., M.d.l.Á.B.-M. and F.J.G.-S.; Software, Y.Y.E.-N.-d.-G. and F.J.G.-S.; validation, Y.Y.E.-N.-d.-G., M.d.l.Á.B.-M., V.M.-M. and F.J.G.-S.; formal analysis Y.Y.E.-N.-d.-G., M.d.l.Á.B.-M. and F.J.G.-S.; investigation, Y.Y.E.-N.-d.-G. and M.d.l.Á.B.-M.; data curation, Y.Y.E.-N.-d.-G.; writing—original draft preparation Y.Y.E.-N.-d.-G.; writing—review and editing, Y.Y.E.-N.-d.-G., V.M.-M. and F.J.G.-S.; supervision, M.d.l.Á.B.-M., V.M.-M. and F.J.G.-S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The corresponding author can provide the data supporting this study’s findings upon request.

Conflicts of Interest

The authors declare no conflicts of interest.

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