Operations Research/Management Science Research in Europe: A Bibliometric Overview

This paper provides a bibliometric analysis of the articles in the ﬁeld of operations research or management science (OR/MS) published in the years 1980–2018 by European researchers. The analysis’s objective is to identify and examine the current state of OR/MS studies in Europe, which publishes about 38% of the papers published worldwide. The analysis was based on the data from the Web of Science (WoS) databases. We found a total of 65,352 papers in 148 diﬀerent journals in the OR/MS ﬁeld. The results provide a general picture of the studies, which are classiﬁed according to the most inﬂuential authors, institutions, papers, and journals. The study revealed that the ratio of OR/MS studies having at least one European author has steadily increased over the decades from 28.27% in the 1980s to 41.29% in the 2010 s. The analysis also provides citation statistics of the European OR/MS articles. The study concluded that the impact of European publications is less than the impact of U.S. publications. The bibliometric analysis of the studies showed that only a small portion of the countries/regions, institutions, and even authors published a substantial portion of the papers, as indicated by the Pareto rule. The research trends have been identiﬁed through an analysis of keyword usage over the years. In keyword analysis, which subcategories are studied together is also identiﬁed. In the paper, collaboration among countries and institutions is also identiﬁed and depicted by using VOS viewer.


Introduction and Literature Review
Operations research and management science (OR/MS) studies are considered an essential part of economic life because organizations are continually looking for the most efficient and productive ways of running their businesses. erefore, the number of OR/MS studies has increased substantially over the last four decades [1]. Even though the initial OR/MS studies were mainly limited to specific regional areas such as the United States and the United Kingdom, the practice of operations research and management science has seen an enormous increase all over the world, including other developed countries, developing countries, and even underdeveloped countries.
Bibliometric analysis, motivated by the development of Internet and online databases, is receiving increasing attention from the scientific community, and the number of bibliometric studies is increasing [2]. Several studies provided bibliometric analysis in many research areas, including OR/MS [1], project management [3], management [4], economics [5], supply chain management [6], and pricing research [7]. However, the number of papers presenting bibliometric analysis in OR/MS has been limited. Merigo and Yang [1] presented a bibliometric overview of research published in operations research and management science in recent decades. e paper's main objective was to identify some of the most relevant research in this field and some of the newest trends according to the information found in the Web of Science (WoS) Database [1]. Laengle et al. [8] presented a bibliometric analysis in OR/MS to identify the most productive and influential universities between 1991 and 2015.
e authors also collected the data from WoS database; however, the analysis is limited to universities and the location of those universities. Chang and Hsieh [9] evaluated the distribution of papers published by Asian authors in OR/MS journals from 1968 to 2006. e authors also compared the impact of OR/MS research in Asia with that of the research in United States and the world. e study also highlighted research trends by analyzing keywords [9]. Bilir et al. [10] provided a bibliometric analysis of the articles in the field of OR/MS published between the years 1980-2017 by researchers from Turkey. e main objective of the analysis was to identify and examine the current state of OR/MS studies in Turkey. e authors also identified the current research trends through a keywords analysis. However, the study was limited to one country [10]. Laengle et al. [11] presented a general overview of the European Journal of Operations Research over its lifetime using bibliometric indicators. ey discussed the performance of the journal compared to other journals in the field and identified key contributing countries/institutions/authors as well as trends in research topics. Even though that study provided valuable information about OR/MS studies, it was limited to papers published in the European Journal of Operations Research.
Our review revealed that there is no bibliometric study of OR/MS research in Europe even though there is one for the overall world and another study for Asia. To bridge that gap in the literature, in this paper, we aimed to provide a general overview of European OR/MS research over the last decades using bibliometric methods and compare the results of OR/ MS research in Europe with that of the world and other regions. We used the WoS as the database for collecting information. Another objective is to identify the research trends in the field in Europe to understand the drivers of the development of European OR/MS studies and to compare the impact of OR/MS research in Europe with that of the United States and the world. Section 2 describes the bibliometric methodology we applied in the analysis. Section 3 contains the results of our analysis. Section 4 summarizes findings and provides suggestion for future research.

Methodology
To conduct the bibliometric study, we reviewed and analyzed articles available in the WoS databases, which comprise the Science Citation Index Expanded (SCI-Expanded), the Social Sciences Citation Index (SSCI), the Arts and Humanities Citation Index (A&HCI), the Science Citation Index (SCI), and the Emerging Science Citation Index (ESCI). e WoS database, which is owned by omson and Reuters, includes studies from a wide range of research areas (252 subject categories), one of which is OR/MS. In the first step, we selected the articles in that category. We then excluded publications from 2019 because that year is not yet complete. As articles from 1975-79 were very limited, we also excluded those years. e list thus included 299,822 publications, including articles, proceedings paper, reviews, and notes. To focus on the most influential of the publications, we selected only articles. us, we reduced the number of publications to 172,535, published in 160 different journals. Because we included only articles in the WoS database, we did not exclude any journal from our analysis.
To further refine the selection, countries in Europe are selected to have a list of articles in which at least one author is from a European country, where a list of 65,352 articles is generated from 50 different countries, published in 148 different journals. In analyzing collaborating countries and keywords, we also used VOSviewer [12] (a scientific software tool for constructing and visualizing bibliometric networks).

Results
We present our research results in the following subsections: number of papers throughout the selected years in Europe compared with the world, leading journals, and most productive authors; citation statistics and most-cited articles; and most productive institutions, collaboration analysis, and keyword analysis.

Number of Papers.
e number of papers published in a specific area is an important indicator in scientific research. To analyze the number of OR/MS papers in Europe, we present the number of papers published in the world and in Europe ( Figure 1). e length of each bar represents the total number of papers published in the OR/MS category in the WoS database. e dark-colored bar represents the number of papers published in Europe, whereas the light-colored bar represents the number of papers published in the world.
ere is an upward trend in the number of papers in the OR/ MS category starting from the 1980 s whose slope increases further after 2003. e number of studies with at least one European author is 65,352 (37.88%). However, the ratio of European studies to studies worldwide has increased steadily over the decades, from 28.27% in the 1980 s, to 32.76% in the 1990 s, to 39.68% in the 2000 s, and to 41.29% in the 2010 s. Figure 2 presents the number of OR/MS papers from the top 25 most productive countries/regions in the world. e results show that one-third of the papers (56,602) have at least one author from the United States. Although the United States dominates OR/MS studies, the ratio of studies from the United States has been decreasing slightly over the decades. China follows the United States with 21,683 studies (12.57%), followed in turn by the United Kingdom (6.84%), Canada (6.40%), France (5.03%), and Taiwan (4.93%).
irteen of the top 25 most publishing countries/regions in OR/MS are European ( Figure 2).
We also analyzed the number of papers from the top eight publishing countries in Europe and the development of the number of papers throughout the years in those countries ( Figure 3) to identify how the number of papers from European countries changes between 1980-2018. Results showed that England is still the leading country in terms of the number of papers published each year; however, the number of papers published from French, German, and Italian researchers are close to the number published by English researchers in recent years. Another conclusion which may be drawn from the is that the number of papers published by researchers from Spain and Turkey was close to 2 Advances in Operations Research  1999 1998 1997 1996 1995 1994 1993 1992 1991 1990 1989 1988 1987 1986 1985 1984 1983 1982 1981 1980 World Europe      at study showed that researchers from European countries mainly preferred journals of European origin (especially the Netherlands, Germany, and the United Kingdom), more so than did researchers worldwide. e journal with the highest h-index among those preferred by academicians was the European Journal of Operational Research (h-index: 211); the one with the lowest index value was the International Journal of Technology Management (h-index: 48). Our study has also revealed that 16 of the 25 most publishing journals are among the best 25% (Quartile 1) of the journals in the field. Table 1 shows the countries/regions in which the articles were published, the numbers of articles, and the proportion of these articles to all articles, their h-index information, the classification made by WoS, and the JRC 2016 Impact Factor values of the 25 journals in which European researchers mostly published.

Most Productive Authors.
We ranked the most productive/influential authors both by number of papers and by number of citations. e author publishing the most papers in the OR/MS category in Europe was Enrico Zio from Politecnico Milano, having published 154 papers. e second-ranked author was Luk N. Van Wassenhove from INSEAD Business School, France, with 122 papers. Wassenhove had two different author name accounts with spelling differences in the WoS database that must be merged. Terje Aven from Stavenger University, Norway, attained the third rank with 116 papers. Table 2 presents most productive authors with their affiliation and number of published papers and ranks them by the total number of citations their papers have received. e most-cited author was Van Wassenhove with 9,684 citations. Van Wassenhove also has the highest H-index value (49), which shows that h number of papers have at least h number of citations. e second-ranked authors by number of citations were Rainer Storn and K. J. Price both from Siemens, Germany, even though they only had one paper in OR/MS.

Citation Statistics and Most-Cited Articles.
By the end of 2018, OR/MS papers in the WoS database with at least one author from a European country had received 1,176,426 citations. We calculated the average number of citations per paper as 18.05. e average number of citations per paper for the articles published by U.S. researchers was 25.12. In comparing the two figures, someone might conclude that European publications had less impact than U.S. publications. Table 3 presents the average number of citations per paper for the top 15 publishing countries.
We also classified the OR/MS articles in the WoS database by the number of citations received. us, the articles that have received most attention by the scientific community have been identified (Table 4). Table 4

Most Productive Institutions. Twenty institutions in
Europe produced more than 500 papers during the years analyzed. e most productive institution in Europe in this field is the Centre National De La Recherche Scientifique, France with 2,492 papers (3.8% of the total). Other institutions with more than 1,000 papers are the Universite Paris Saclay Comue, France and Eindhoven University of Technology, Netherlands. Table 5 lists the top 20 institutions, their number of papers, and the percentage of the total. e results show that around one-fourth of the papers are published by only 20 institutions, 15 of which are located in only three countries: the United Kingdom, Netherlands, and France.
3.6. Collaboration Analysis. As part of the bibliometric analysis, we investigated the collaboration among authors and the collaboration among countries in the field of OR/MS studies in Europe. First, we analyzed how the average number of authors changed from 1980 through 2018. en, we presented the most collaborated countries within Europe and non-European countries collaborating with European researchers.
In the analyzed papers, the average number of authors per article was 2.52. is implies that researchers in the field have been collaborating more and more through the years. Table 6 provides detailed statistics relating to the number of authors per article. More than 34% of the articles were written by two authors, around 27% of the articles were written by three authors, and around 20% of the articles were single-author papers. e maximum number of authors per article was 22. Only 54 articles in the OR/MS field had 10 or more authors in Europe.
We also used data retrieved from the WoS database to show the number of different countries involved in collaborative research articles. In Table 7, first the number of countries whose researchers collaborated on an article was calculated, with the percentage of each listed. at is, 62.65% of the articles had authors from only one country, 28.54% of the articles had authors from two different countries, 7.20% of the articles had authors from three different countries, and so on.
We also analyzed the data to calculate the number of articles between (or among) collaborating countries/regions (Table 8) using the address field in the data retrieved to define collaborating countries/regions. e most collaboration occurred between British and Chinese authors with 482 articles.
is is followed by Turkish and American authors with 450 articles. e United States was the most likely to collaborate with European countries in the field of OR/MS.
We also examined collaborations between European and non-European authors. Table 9 lists only non-European countries with more than 500 authors. e United States took first place with 6,227 articles (9.5%), followed by China (2,293 articles, 3.5%) and Canada (2,006 articles, 3.1%).
We also used collaboration data to depict the relationships between the countries of collaborating authors with VOSviewer, a bibliometric network visualization tool (VOSviewer Visualizing scientific landscapes) to create "Collaborated Countries/Regions Relationship Diagram," as shown in Figure 5. In Figures 5 and 6, the size of the have relatively higher numbers of collaborations. e figure also shows that European countries collaborated with non-European countries as well as they did with European countries.
VOSviewer also allowed us to highlight the strength of collaboration between one selected country and other countries by pointing to that selected country with the mouse. Figure 6 shows countries collaborating with British researchers. e thickness of links indicates that British     Average # of authors

Keyword
Analysis. e WoS database includes two different types of keywords: "Author Keywords" and "Keywords Plus." Some articles discuss which of them is better (e.g., [13]). While "Keyword Plus" yielded more keywords, "Author Keywords" better explained the content of the article because the author(s) selected them specifically.
is research analyzed and included "Author Keywords" from 1990 through 2018 in the resulting tables.
To do the keyword analysis, we first retrieved the author keywords from the WoS database and corrected spelling mistakes. Later, we changed some British spellings of words to American English, such as optimization and optimization. We deleted dashes or spaces that we thought unnecessary (e.g., "multiobjective" or "multiobjective" to "multiobjective") and examined in detail keywords repeated more than five times. en, we identified and merged keywords with the same or similar meanings, such as "MCDM" and "Multi Criteria Decision Making" or "AHP" and "Analytic        1990 and 2018). Keywords 1990Keywords 1991Keywords 1992Keywords 1993Keywords 1994Keywords 1995Keywords 1996Keywords 1997Keywords 1998Keywords 1999  Hierarchy Process." In addition, we identified similar keywords with same the meaning, such as "Heuristic," "Heuristic Algorithm," "Heuristic Algorithms," "Heuristic Method," "Heuristic Methods," "Heuristic Optimization," "Heuristic Search," and "Heuristics." ey were basically the same areas of study used by different articles, so we changed them all to Heuristics. In the analysis, we eliminated no keywords; we only examined them and changed, corrected, or merged them to maintain a common language among articles. Table 10 depicts a list of the number of repetitions of the top 20 keywords, repeated 500 or more times between the years 1990 and 2018 (inclusive). e most repeated keyword is "Multiobjective Optimization" (1,900 times), followed by "Heuristics" (1,587 times) and "Scheduling" (1,577 times). e table also shows how the number of appearances of each keyword changed throughout the years analyzed.

Advances in Operations Research
We then generated mini column graphics based on numbers of keywords and on percentage of each keyword, calculated by dividing the number of keywords into the number of articles each year. Figure 7 shows these mini column graphics. e mini graphics show that the frequency of "Multiobjective Optimization" increased in interest over the years studied. Although the frequency increased slightly percentagewise, it did not increase as much as did the keyword frequency. Almost no interest existed for "Supply Chain Management" before 2000 or for "Metaheuristics" before 2004. Considering the number of keywords by years in number, we can conclude that no substantial change in the number of articles with the keyword "Linear Programming" existed. However, the trend for the percentages of articles through the years with the keyword "Linear Programming" decreased.
When we compare the most repeated keywords in our study and keywords in similar studies (e.g., [1,9], we do not find much difference between the keywords in the studies in Europe and the studies worldwide or in Asia. at is, the European OR/MS studies have similar keywords and

Advances in Operations Research
subresearch categories to those studies worldwide. e results also support the common belief that in academic life, some subjects become trendy, studied by researchers study heavily for a period of time (as in "Tabu Search" and "Metaheuristics").
Using the VOSviewer software, we investigated the original table of keywords analyzed and the 100 most repeated keywords for further consideration to visualize the keywords studied together.
at visualization also helped us understand which subcategories of OR/MS are commonly studied together in Europe. We manipulated the above-mentioned data with VOSviewer to create "Keywords Relationship Diagram" in Figure 8, which presents only categories that have been used 25 or more times.
In the diagram, the size of the circle for each keyword represents the number of articles using that keyword while the thickness of the links represents the number of articles that used the keywords in both sides of the link, similar to the network of collaborating countries/regions in Figure 5.
Drawing conclusions from Figure 8 was somewhat complicated because it included 100 keywords. To make it easier to understand the relationships with mostly used keywords, we highlighted the most repeated keywords, as in Figures 9(a)-9(c). e diagram clearly illustrates which methodologies are most commonly jointly used with "Multiobjective optimization" in the analyzed papers. e diagram indicates that "Multiobjective optimization" is mostly used together with "Decision support systems," "Genetic algorithms," and "Evolutionary algorithms." Figure 9(b). Keywords relationship diagram of "Heuristics" presents the same diagram for "Heuristics." e diagram indicates that "Heuristics" is mostly used together with "Scheduling" and "Combinatorial optimization." e same diagram is presented for "Scheduling" in Figure 9(c). Keywords relationship diagram of "Scheduling." is diagram also lists the scheduling studies with the number of appearances in the literature (production scheduling, production planning, maintenance scheduling, routing etc.) e diagram also shows that "Scheduling" is mostly used together with "Heuristics."

Conclusion
is paper provides a systematic bibliometric review of OR/ MS studies in Europe. Although some bibliometric analysis has been done on OR/MS studies worldwide, this study is the first paper to focus on Europe, which publishes about 38% of the papers published worldwide in the analyzed subject. In conclusion, along with the complete overview of the studies, we also present some major and interesting results. 3.9%) were among the top five journals in which researchers published at the highest rates; 30.85% of the total publications appeared in those five top journals. e analysis also revealed that the preferences of European researchers differ from the non-European researchers. Researchers from European countries mainly preferred to publish in journals of European origin.
We calculated the average number of citations per paper as 18.05. Compared with the average number of U.S. citations in the same subject (25.12), we have concluded that the impact of European publications is less than the impact of U.S. publications. However, even in Europe, that ratio varied from author to author, from institution to institution, and, surprisingly, from country to country.
Our bibliometric review revealed that substantial portions of the studies were written as joint efforts with academicians working in other countries, either within Europe or outside Europe. e increasing number of average authors per article implies that the researchers in the field have been collaborating more and more through years. In the analyzed papers, the most frequent collaboration occurred between British and Chinese authors (482 articles), followed by Turkish and American authors with 450 articles. European authors collaborated the most with American authors (6,227 articles), followed by European authors collaborating with Chinese authors (2,293 articles). e collaboration analysis results also showed that European researchers collaborated with non-European countries as much as with European countries in the field of OR/MS. Analysis on the most productive institutions revealed that 20 institutions produced more than 500 papers during the years under study. e results also show that one-fourth of the papers were published by only 20 institutions. Analysis of the papers showed that only a small portion of the countries, institutions, and even authors published a substantial portion of the papers in the field of OR/MS in Europe, as indicated by the Pareto rule. e keyword analysis indicated that the three keywords that frequently appeared in 2000-2018 were "Multiobjective optimization," "Heuristics," and "Scheduling." When we compared the most repeated keywords in our studies with keywords in similar studies, the results did not show much difference between the keywords in the European studies and those from other parts of the world. e results also support that, in academic life, some trendy subjects are heavily studied for a short period of time.
e bibliometric approach used in our study has one major limitation: it examined the OR/MS studies published only in OR/MS journals. Indeed, scholars do not necessarily publish their papers only in OR/MS journals indexed under the WoS. However, no practical way exists for identifying all OR/MS articles published in other journals or indexed by Scopus, ProQuest, or other indices.

Data Availability
e data used in the analysis are open to public who have access to Web of Science databases. We can also provide the data we gathered from WOS databases upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.