Thomas Kuhn's idea of scientific revolutions (Kuhn 1962) and the concept of paradigm shift made many researchers curious about the intellectual structure of their academic fields. Studies of the intellectual structure of an academic discipline provide ways to examine the discipline's current state as well as to show the changing structure as the field goes through modification in their subfields or subject specialties. In particular, the field of Library and Information Science has been using bibliometrics such as citation analysis in analyzing scientific literature and applying it to create a visualization of an academic discipline's underlying structure. As McCain (1991a, 290) mentioned, citation analysis can “identify the ‘core’ literature of various fields of scholarship as represented in their journal literature and study synchronous and diachronous patterns of literature aging.” The analysis can use individual documents, authors, or journals as the unit of analysis, and various methods for examining patterns of citations such as co-citation, inter-citation, and bibliographic coupling.

Influenced by Kuhn's structural view of scientific development, Small (1973) introduced the co-citation analysis as “a new measure of the relationship between two documents” in the scientific literature, and later applied the methodology across all the sciences “so that the full structure could be viewed on a single map” (Small 2003, 395). Unlike the basic citation analysis or bibliographic coupling examining the relationship between the citing unit and the cited unit, the co-citation analysis provides a dynamic view of a discipline's structure because it examines “the frequency with which two items of earlier literature are cited together by the later literature” (Small 1973, 265) and the result of co-citation analysis may vary dependent on the studied period. In other words, it can also be used as a means of investigating the scientific growth of a discipline by studying the changing structure of an academic discipline over time.

The purpose of this study is to investigate the intellectual structure of the library and information science field by identifying core journals of the field as well as their subject fields. Co-citation patterns of journals will be analyzed using network analysis, and subject categories of the journals will be used to help examining the intellectual structure of the field as well as discerning which subject specialties are related to the core journals.

Studies of the intellectual structure have been done in various academic fields, including economics (Kwak and Chung 2012), marketing research (Yoo et al. 2012), geography (Sluyter et al. 2006), biomedical informatics (Jeong and Kim 2010), and stem cell research (Zhao and Strotmann 2011), and library and information science (Yoon and Seo 2001; Yoo 2003; Moya-Anegon, Herrero-Solana, and Jimenez- Contreras 2006; Park 2013) and in subfields of Library and Information Science such as information retrieval (Rorissa and Yuan 2012), bibliometrics (Lee and Choi 2011) and diffusion of innovation (Kim and Park 2011). Especially, the intellectual structure of library and information science has been examined with diverse methodologies, including co-citation analysis and bibliographic coupling, combined with statistical approaches, such as cluster analysis and MDS. In a study comparing existing techniques with a new technique for using topics as a measure of relatedness, Lu and Wolfram (2012) reviewed five representative techniques for mapping bibliometric units; direct citation, bibliographic coupling, cocitation analysis, co-authorship analysis, and co-word analysis.

Yoon and Seo (2001) investigated the intellectual structure of Korean library and information science in 1990s using author co-citation analysis. They analyzed author co-citation data from three library and information science journals from 1990 through 1999, using MDS, cluster analysis, factor analysis, and crosstab analysis. MDS was used for mapping of 50 authors with a correlation coefficients matrix but not successful in distinguishing the relationships between authors because of the weak correlations tendency between authors. Cluster analysis was performed to generate clusters of authors with similar characteristics, and it yielded 13 distinct clusters representing subfields of library and information science, such as library management, information retrieval, reference service, cataloging and classification, bibliometrics, and bibliographies. Factor analysis was used for confirmation of the cluster analysis result and revealed that the result of the factor analysis and the cluster analysis were quite well matched. Lastly, crosstab analysis was used for investigating the interrelation between authors and their corresponding subject fields. According to the result of their crosstab analysis, library management and information retrieval were two of the most highly cited subject fields among 20 subfields of library and information science. Among other things they found out from this study, changes in the intellectual structure of library and information science seemed to be brought by social and technological changes.

Yoo (2003) investigated the intellectual structure of library and information science by clustering the index terms used in the LIS journals. The index terms were clustered based on the linkage of the terms and the co-occurrence frequency. The result indicated that KORMARC, information retrieval, public libraries, OPAC, and university libraries constituted the highest level clusters, which showed cataloging was still an important subject in the library and information science research. The study also investigated the very first used index terms of each year during the period between 1978 and 2001 and examined the frequency of each “first used” index term appeared in journal documents. Through the examination, the study found out that ‘public libraries’ were the most frequently appeared index term in 1978, and other types of libraries were also appeared as important research topics during those days. Some cataloging-related terms, such as OPAC and AACR, were frequently used in the late 1970s and early 1980s, and studies of information users started to appear in the mid 1980s. Specialty databases including MEDLINE started to appear in the index terms in 1990 and the term internet was obviously the most highly used as well as digital libraries in 1995. As a very logical and natural development, the term ‘document delivery’, ‘metadata’, and ‘retrieval engine’ were used as research topics in 1996, and research in 2000 showed changes of research trends and rise of other aspects of the field, such as ‘record management’ and ‘knowledge management’. As the researcher admitted, the indexing term-based analysis method was not very successful in explaining the intellectual structure of library and information science, but it confirmed that the intellectual structure of the traditional classification system served the purpose of classifying library materials.

There are other approaches to define the intellectual structure of information science by formulating a knowledge map (Zins 2007a/2007b; Zins and Santos 2011). Using a methodology called “Critical Delphi” with a panel of 57 scholars from 16 countries, the study incorporated reflections regarding schemes of classifying the knowledge structure of information science from 28 panel members. Some scholars used abstract and theoretical descriptions for each category, such as “epistemological”, “systematic”, “cognitive”, “psychological”, “communicative”, and “technological”, and others used more concrete and practical descriptions, including “information retrieval”, “information management”, “knowledge organization”, “user studies”, and “quantitative analysis in IS.” Although the study does not provide “one and only” gigantic scheme of information science, it does show how diverse the field of information science can cover as its topics with rationales provided for each scheme. With the same methodology and the panel, Zins (2007b) suggested a conceptual model of information science, composed of two main groups. The first group had one category, “foundations”, and the second group had nine categories, including “resources”, “knowledge workers”, “contents”, “applications”, “operations and processes”, “technologies”, “environments”, “organizations”, and “users.” The “Critical Delphi” methodology let the researcher formulate the knowledge map using three phases, starting from systematic conceptions of data, information, knowledge, and information science, and then the panel critically reflected on the conceptions, which lead to the construction of the knowledge map as a result. The first group represents the metaknowledge of the field or philosophical grounds of information science and the second group’s categories are considered as nine basics of information science. Nguyen and Chowdhury (2013, 1235) also used knowledge mapping to help researchers in “exploring and understanding the DL (digital library) knowledge domain and its evolution” and constructed a knowledge map of digital library research with 21 core topics, including social web, semantic web, mobile technology, and information retrieval, as well as 1,015 subtopics covering a period from 1990 to 2010. The research indicated that the digital library research topics came from various disciplines, including library and information science, computer science, knowledge management, and the social sciences. Lee et al. (2010) also investigated the digital library research field using journal and descriptor profiling methods, and generated maps with distinctive journal clusters and descriptor clusters. The results showed 11 descriptor clusters with representative journals, including knowledge management, online cataloguing formats, online information retrieval, world wide web, online reference work, and distance learning.

McCain (1991a/1991b/1998) introduced co-citation analysis using journals as the unit of analysis, instead of using individual documents or authors. In author co-citation analysis, “the raw data are counts of the number of times any two authors are cited together in later works” (McCain 1991a, 290). The co-citation patterns reveal major subject specialities of a discipline as well as central and peripheral authors within each subject specialty and the discipline. Journal co-citation analysis uses the same logic behind other co-citation analysis, which means that “a journal’s placement (central or peripheral) in the map should reflect the congruence between the content of that journal’s articles and the research interests of authors publishing (and citing) in the field represented by the set of journals studied” (McCain 1991a, 291). McCain applied journal co-citation analysis in an attempt to map Economics (1991a) and Genetics (1991b), and resulted in “clear, coherent, and reasonable” (1991a, 295) mapping of the fields. In the study for generating cocitation maps for Genetics, McCain (1991b) used journal cocitation patterns to compare the results with the core network analysis, which identified important journals using “the strong patterns of citations made and received by journals.” The study indicated that such bibliometric analyses using journals were helpful to “identify a manageable number of important journals at both the disciplinary and subdisciplinary levels” (McCain 1991b, 326) and journal co-citation analysis portrayed the subject structure of a discipline, offering collection specialists “the ability to map a reasonably representative set of journals rapidly, modify the map by adding additional titles, and identify journals likely to be of use to various constituencies represented by the clusters” (McCain 1991b, 326). Later, McCain (1998) used journal co-citation analysis in neural networks research, employing cluster analysis and MDS maps to identify foundation research areas in neural networks research.

Moya-Anegon, Herrero-Solana, and Jimenez- Contreras (2006) used the self-organizing map (SOM), clustering, and multidimensional scaling (MDS) to visualize the field of library and information science, based on author co-citation analysis (ACA) and journal co-citation analysis (JCA). Using 17 journals selected for the study based on impact factors in Journal Citation Reports 1996 edition, the ACA results showed 6 main domains in information science research, including scientometrics, citationist, bibliometrics, communication theory, soft information retrieval, and hard information retrieval. The MDS map of journal co-citation analysis showed 4 main clusters, including information science, library science, science studies, and management. Their result confirmed the subject domains identified in previous domain analysis studies (White and McCain 1998) and suggested the use of network analysis for other kinds of representations.

Åström (2010) investigated the relationship between library science and information science by comparing knowledge maps created for each field with author co-citation analysis and journal co-citation analysis, and produced a joint map for library and information science. The result showed a networked map of 37 most-cited journals with some journals in the center and others in the periphery. Journal co-citation analysis is also used for identifying core journals in other areas of research. Jo and Lee (2012) identified core journals for pharmaceutical researches in Korea using journal citation frequency analysis and journal co-citation analysis. They used factor analysis on journal correlation matrix which resulted in 116 core journals with 18 subject domains.

The data for this study was collected using Social Science Citation Index from Web of Science. Thirty journals were selected based on the Journal Impact Factor in 2012 Journal Citation Reports Social Science Edition for Information Science & Library Science as the subject category. The list of journals with the abbreviated and full title is shown in Table 1. First, raw counts for “cited references search” results for each journal were recorded. Initially, timespan for the search was set as “All Years” but later changed into “from 2011 to 2013” because the initial search yielded too many results for most of the journals. The selected two-year period yielded sufficient but not too many number of cited references, based on subjective judgment by the researcher. Because of the limitations that restrict the first 500 cited references to be included in the final result, too many results may not be suitable for distinguishing highly cited references. Table 2 shows the comparison between raw “cited references” counts when all years were counted and when references since 2011 were included. For the same reason, the language of the references was restricted to English, and the type of references was limited to articles.

The cited reference search shows the list of cited references with the particular journal as the “cited work”, and all of the results should be selected to get the final set of references that are citing the cited references. The search history page shows the number of references, or articles, that cite the journal’s articles. The co-citation counts were obtained by “AND”ing the search results for each pair of journals at the search history page. For example, there were 152 references that cited articles in MIS Quarterly and Information Technology & Management, but only 9 references cited articles in MIS Quarterly and Journal of Health Communication. The co-citation counts were used for constructing a 30 by 30 matrix, consisting of co-citation counts for each pair of journals. It was a symmetrical non-directional matrix, with diagonal values set at 0.