A Bibliometric Analysis of Global Online Marketing Research Trends

The revolution of the internet technology has increased the demand for online marketing strategies. Over the years, online marketing has received considerable attention owing to its advantages to both individuals and organizations. This study sought to analyze the global online marketing research trends from 1975 to 2019 through a bibliometric analysis procedure. A total of 789 published articles were retrieved from the Scopus database for the analysis. The results suggest that between 20012010, Google AdWords, affiliate marketing, e-mail marketing, search engine advertising (SEA), search engine marketing (SEM), search engine optimization (SEO), social media marketing, and convergent mobile technologies is the most preferred online marketing platform in the hospitality, tourism, hotel, multi-national corporations (MNCs), construction, and software industry. Between 2011-2019, online marketing study focused more on social media marketing platforms such as Facebook, Instagram, Twitter, blogs, and dual-channel (i.e., YouTube). Also, the online marketing trend switched its focus from multi-national corporations (MNCs), construction, and software industry in 2001-2010 to small and medium enterprises (SMEs), business-to-business marketing, and agribusiness in 20112019. The findings of this study provide useful information to academic researchers and industry practitioners to aid their understanding of online marketing research development and online marketing roles in strategizing marketing plans. This paper innovates by identifying new areas for investigation and potential research opportunities in the field of online marketing. It is one of the pioneer studies which provides a greater understanding of online marketing research by investigating its evolution through bibliometric analysis.


Introduction
The revolution of the internet technology has increased the demand for online marketing. It also enhances global e-commerce growth to an unprecedented level (Cao et al., 2018). As online marketing naturally involves a lower cost than traditional marketing platforms like face-to-face salespeople or intermediaries and distributors (Andreki and Yazdanifard, 2014), many organizations start to adopt online marketing to increase their customer base globally (Durmaz and Efendioglu, 2016). In contrast with traditional marketing, online marketing has become a powerful marketing platform for advertisements (Schwarzl and Grabowska, 2015;Yasmin et al., 2015). Several advantages that online marketing could offer beyond traditional marketing include complementing the current needs of every user, bringing closer digital relationships between individuals and organizations, and allowing businesses to provide information at anywhere and anytime (Wilson and Makau, 2018). Online marketing is also understood as e-marketing, where it markets products and services over the internet (Chien and Lee, 2018). Previous studies suggested that the applications of online marketing consisted but not limited to search engine optimization, e-mail marketing, and social media marketing (Adetunji et al., 2018;Giomelakis and Veglis, 2016;Karjaluoto and Taiminen, 2015). Nevertheless, online marketing, such as affiliate marketing, has not received enough attention in recent times. Affiliate marketing is a type of online marketing platform where a company or merchant signs an agreement with the third party that acts as an affiliate to feature a link from its website on the affiliated sites (Duffy, 2005). While there has been a growing interest in online marketing platforms, very few studies were found to evaluate and analyze scientific publications from a global perspective. He et al. (2012) offered research analysis on e-marketing for 10 years (2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010), whereas Kim and McMillan (2008) focused specifically on the relative youth of internet advertising field from 1994 to 2003. Both studies used Web of Science (WOS) (previously known as Web of Knowledge) as the source of data mining. Although WOS and Scopus databases have a high association (i.e., overlapped in journal indexing), they indexed different journals (Öchsner, 2013). Scopus is recognized as the largest abstract and citation database for peer-reviewed journals and equipped with smart tools to track, analyze, and visualize research publications (Hoogendoorn, 2008). Therefore, based on the above discussion, the present study attempts to use the Scopus database as an effort to discover more topics that may be excluded in WOS and may not have discussed by He et al. (2012) and Kim and McMillan (2008) in their studies. As such, this study analyzed 789 research articles related to online marketing in the past four decades via bibliometric approach. This paper will be beneficial for individuals, policy makers, and researchers to understand the research trends in online marketing and to provide some recommendations for future research. This study provides the following list of objectives: 1. To analyze chronological distribution patterns of the online journal articles. 2. To identify the most productive journals. 3. To reveal the most productive countries and academic institutions. 4. To show the contributions of the prolific authors. 5. To highlight popular research topics and sub-themes of the current trend. 6. To propose research opportunities for future research.

Research Methodology
A bibliometric analysis study is a quantitative approach to analyze global research trends in a specific area or topic based on published academic journals or articles (Abbott et al., 2018). This way of approach differentiates a bibliometric analysis paper from a review paper where bibliometric paper intends to discuss the trend, latest development, and future direction of a specific area of study.

Data Source and Search Strategy
Scopus database was used for data mining in the present study. The central themes established in this study are research journals and articles that contain keywords of "online marketing", "emarketing", and "affiliate marketing" in their titles and abstracts. The oldest publication was traced back to the year 1975, and the most recent ones were in 2019. The query string used for the search was: (TITLE-ABS ("online marketing" OR "e-marketing" OR "affiliate marketing")) AND EXCLUDE (PUBYEAR, 2020)) AND (LIMIT-TO (DOCTYPE, "ar")) AND (LIMIT-TO (SRCTYPE, "j")). This query string resulted in 836 documents. To ensure that there is no review paper captured in our analysis, additional steps were included in the query string, which resulted in 125 articles irrelevant to our analysis. These articles contained terms like recent, progress, review, critical, and highlight in the titles and abstracts. After the screening process, we discovered another 50 articles that were categorized as review articles. All their EIDs were retrieved and added into the search query so that these review articles would not appear in the results later. The final query produced 789 researchbased articles. To understand the search strategies, one should note that the best way to get the most accurate data from an author is to use the author's ID (Scopus field code: UA-ID). The author's profile should present a list of all the names in one single profile. Next, the search results will then be limited to a country using the field code AFFILCOUNTIRE to access single-country publications (SCP). The search results of the central themes were analyzed based on year, source, author, affiliation, country/territory, subject area, and document type. As for ranking purposes, bibliometric indicators such as total publications (TP), total citations (TC), and CiteScore were used. The details of the search strategies and search strings used in this study are provided in the appendix.

Bibliometric Maps
To establish and visualize the bibliometric maps, indicators like citations, bibliographical information, and author keywords of 789 articles were exported to VOSviewer (version 1.6.13, Centre for Science and Technology Studies, Leiden University). The maps constructed using VOSviewer must include all the items required to be analyzed. The items are referred to as the countries or author keywords. There may be a relationship between two items or between any pair of items, and each link of the items is represented by a positive numerical value. A strong link is an item with a higher numerical value. For example, in co-authorship links, the link strength reflects the number of publications that two researchers have co-authored. In contrast, the total link strength specifies the total strength of the co-authorship links of a given country with other countries. Similarly, in co-occurrence links, the link strength between author keywords indicates the number of publications in which two keywords occurred together. Further explanations could be referred to as the VOSviewer Manual (Van Eck and Waltman, 2019) for more details.

Analysis of Co-authorship
In the co-authorship analysis, 77 countries associated with 1857 authors were selected for data analysis. The affiliated countries/territories were clustered into 6 continents, namely Balkans, Africa, Europe, Oceania, America, and Asia.

Analysis of Co-occurrence
To discover the changes in research themes, the forty-five-year period was divided into three subperiods: 1975-2000, 2001-2010, and 2011-2019. The present study gathered all keywords from the 789 published articles to perform co-occurrence analysis. The visualized keyword networks were established to display the relationship between keywords in each sub-period. The yellow color of the keyword indicates that they were the research hotspots during each sub-period (see Figure 7 and 8).

Results and Discussion The Output of Publications by Year
A total of 789 research articles were published over the last 45 years, as displayed in Figure 1. The first publication was published in 1975 (Bailey and Tierney, 1975), and there was no publication reported until 1988 (English-Zemke, 1988). A clear interest in online marketing research emerged in 2000, resulting in a gradual increase in publications from the year 2000 onwards. Even though the number of articles was increasing at a different rate each year, its overall growth rate was reported at 21.4%. However, the number of annual publications of online marketing research after 2000 has increased tremendously in an exponential manner, as illustrated in Figure 1. It is expected that the number of yearly publications will continue to increase beyond 2019. Moreover, there is a consistent upward growth in the use of terms such as "online marketing" and "e-marketing" in the literature. Specifically, the term "affiliate marketing" was discovered to appear in research articles at the beginning of 2010 (Fox and Wareham, 2010). Over the past 10 years (2010-2019), the term "affiliate marketing" was not gaining much attention from the researchers, and its publications were minimal compared to "online marketing" and "e-marketing" (see Figure 1). According to the Scopus database, only 14.1% (111 articles) of the online marketing papers were published in open-access journals. Some of the articles were not freely accessible online, and users have to pay a certain amount of fees to access the materials. Hence, it is suggested that the researchers should opt to publish their papers in open-access journals to increase accessibility and number of citations.

Analysis of the Subject Areas
There are various areas of study in the field of online marketing research. Figure 2 represents the bibliometric analysis of the top five subject areas in online marketing research. The analysis on the subject areas indicates that business, management, and accounting (437 articles) are the primary focus in online marketing studies, followed by social sciences (210 articles), computer science (186 articles), engineering (103 articles), and economics, econometrics, and finance (88 articles).

Top Productive Journals
Our results indicate that there are six different publishers in the list of top ten most productive journals (see Table 1). The top ten productive journals include International Journal of Internet Marketing and Advertising (10, 1.3%), International Journal of Recent Technology and Engineering (10, 1.3%), Journal of Marketing Management (9, 1.1%), Marketing Intelligence and Planning (9, 1.1%), Internet Research (8, 1.0%), Computers in Human Behavior, European Journal of Marketing (7, 0.9%), Journal of Interactive Marketing (7, 0.9%), Journal of Internet Commerce (7, 0.9%), and Advanced Science Letters (6, 0.8%). According to the CiteScore report in 2019, three of the journals were having CiteScores above 5 (i.e., Internet Research (6.73), Journal of Interactive Marketing (6.62) and Computers in Human Behavior (6.14)). Although the Journal of Interactive Marketing was ranked 8 th with 7 articles, its total number of citations was significantly higher than the other journals.  Figure 4 shows the top 15 most productive countries and academic institutions that contribute to the growth of online marketing research globally. About 61.6% of the global publications were added by the United States, United Kingdom, China, and India, indicating that these four countries are the main contributors in online marketing research. The United States was the leading country among the others with 188 publications. The United Kingdom was ranked behind the United States, with 82 publications followed by the rest, as shown in Figure 5. Among the 15 countries, only India, Taiwan, and Indonesia recorded 80% and above of the single-country publications (SCP). This finding explains that these three countries have strong intra-country collaborations. In contrast, South Korea was found to have the least SCP at 38.9%, where only 7 out of its 18 publications were linked with multiple affiliations from 6 different countries. As explained by Glänzel et al. (1999) and Polyakov et al. (2017), global research collaborations typically contribute to greater citation impacts and improve research quality. Nonetheless, in the category of academic institutions, there are two universities listed in the top 100 best universities based on QS World University Rankings 2019 (QS World University Rankings, 2019). They are Fudan University (ranked 44 th ) and Purdue University (ranked 100 th ). This result demonstrates that the online marketing field has received attention from the top universities in the world. Further, the distribution of countries/ territories per region is illustrated in Figure 5. The highest number of countries per region came from Africa (33), followed by Asia (21), Africa (9), America (7), and Oceania (2). The results of co-authorship analysis show that United Kingdom was the most affiliated country where it is linked with 29 countries/ territories with 51 times of coauthorship.    Table 2 lists the 10 most prolific authors in online marketing research. Most authors were having approximately the same number of publications. Chong, Law, Tsao, Xiang, and Yan had published 4 research articles, whereas the rest with 3 publications each. The author, with the most citations, is Xiang, Zheng (1153 citations). The majority of the authors were affiliated with universities from the United States and Slovakia.

Author Keywords Analysis
As for author keywords analysis, it was divided into three different periods. 13 articles were published between 1975 and 2000, which created 22 specific keywords. On the other hand, 185 articles containing 525 unique keywords were reported between 2001 and 2010. Finally, 591 articles with 878 specific keywords were published between 2011 and 2019. Figures 6, 7, and 8 display the three visualized keyword networks. The thickness of the line shows the strength of the co-occurrence keywords. Different areas of research interest were discovered and presented in the three figures. First, there were more keywords and research areas in the second and third sub-period than the first one, indicating that online marketing research was growing over time. Second, the term "online marketing" only appeared in 2001-2010 and 2011-2019, suggesting that the term "online marketing" was not famous before 2001. Third, from 1975 to 2000, online marketing research did not focus on any specific platform or emerging trend, while between 2001-2010, Google AdWords, affiliate marketing, e-mail marketing, search engine advertising (SEA), search engine marketing (SEM), search engine optimization (SEO), social media marketing, and convergent mobile technologies emerged as specific research platforms in the areas of hospitality, tourism, hotel, multi-national corporations (MNCs), construction, and software industry (see Figure 7). Fourth, online marketing study focused more on social media marketing platforms from 2011 to 2019, such as Facebook, Instagram, Twitter, blogs, and dual-channel (i.e., YouTube). Nevertheless, online marketing research brought various platforms such as electronic catalogs, mobile marketing, affiliate marketing, SEA, SEO, and SEM, and Google AdWords as joint research in 2011-2019. Finally, the online marketing trend switched its focus from multi-national corporations (MNCs), construction, and software industry in 2001-2010 to small and medium enterprises (SMEs), business-to-business marketing, and agribusiness in 2011-2019. Hospitality, tourism, and hotel sectors remained as the research hotspots in both sub-periods (see Figure 8).