The ISI file format was developed by the Institute for Scientific Information (ISI) for its first computer-based citation indices. It tags document data (names of authors, titles, source titles, addresses, publication year, start and end pages, and so forth). The ISI format has developed over the years so that it now incorporates abstract information, keywords for articles derived from their references, an ever-developing set of subject categories and so forth. The great advantage of the ISI format, however, is that files with this formatting can be read directly by a number of specialist software packages written for bibliometric analysis. In addition to the ISI format, there are other specific formats for information downloaded from Scopus, and this format is also now generally readable by citations data software.
The analysis of Bibliographic Coupling relationships, first described by Kessler (1963), recognises that two documents can be said to be bibliographically coupled if they share a reference to a common third document (Box 5; Figure c). The degree or scale of bibliographic coupling between any two documents can be said to increase as the number of shared references increases. The analysis of bibliographic coupling relationships makes the assumption that two research outputs that make a knowledge claim about the same specific research question are both likely to share a number of references of recent date to earlier published knowledge outputs discussing this same research question. Bibliographic coupling networks link together documents or authors within a relatively narrow time frame – the active life of the research problem. In effect, therefore, the degree of bibliographic coupling increases among documents that form part of the same active Research Front within a discipline or domain (see Persson 1994).
The analysis of Co-Citation relationships, first described by Small (1973) and Marshakova (1973), recognises that two documents (or sources, or authors) can be said to be co-cited when they both appear in the reference list of a third document (Figure c). The degree of co-citation between any two documents is a reflection of the number of instances in which their co-citation occurs. In contrast to bibliographic coupling, co-citation has been shown to reveal a pattern that looks further back in time. Co-citation analyses of documents, authors and sources identify the source of the broad base/fundamental research knowledge within a discipline or smaller research domain. Co-citation networks tie together documents, sources or authors that may be separated by many years or decades. The mapping of networks of documents tied together by co-citation relationships, therefore, allows us to see the Intellectual Base or Research Core of a discipline or specialty (see Persson 1994). In the same way, we can think of networks of co-cited authors as another view of the research core but from the perspective of its writers.
For any single document, WoS allows subscribers to download the full range of publication data as contained in the citation index, as well as basic data on the cited documents (the list of references) as plain text files following the ISI format (See Box 3). Data on the list of archaeological research outputs was downloaded in files with batches of up to 500 document records (the maximum number allowed in a single download) and then edited into a single file retaining the same formatting. The final file contained the full publication data on the 20,339 documents published between 2004 and 2013, along with basic data (author, date and source name) on more than 270,000 cited documents published during this period and in many cases much earlier. These data were used for mapping and analysis using specialist bibliometric software.
Discipline maps are usually based on one of two different forms of citation analysis: bibliographic coupling or co-citation (See Box 4). Research has shown that analyses based on co-citation and bibliographic coupling relationships identify two different aspects of research-related networks of knowledge: the current active Research Front, identified through bibliographic coupling, and the Intellectual Base (or Research Core) identified by co-citation analysis (Persson 1994). In scientometric research, a Research Front (sensu Price 1963) is defined as a single problem or a related set of problems that are the current focus of active research; the Intellectual Base is defined as the essential knowledge necessary to undertake research within a specific research domain. The Intellectual Base, therefore, contains the significant or classic works that have given shape or meaning to a broad area of knowledge or enquiry and it will include published works from the present as well as works published years or decades before. By contrast, the Research Front is usually defined by works that are contemporary or just a few years old and of importance to those researchers working on the same problem. For this study, co-citation analysis of sources and authors has been used to map the Intellectual Base of archaeology.
In addition to these maps of the Intellectual Base, bibliographic software has also been used to construct a network map (Figure 6) of discipline terms extracted from document titles and/or abstracts. This network of terms represents the vocabulary of a discipline whose understanding is necessary for the comprehension of its literature. In many cases this network of terms will also identify the concepts of a discipline and, therefore, the theoretical understanding required by a disciplinary practitioner. Depending on the time frame of the bibliometric data used, a network of terms may relate to the research domain as a whole or to the changing research fronts over a period.
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