Citation databases include information on publications in the database and the citations between them. Citation information shows how often other publications in the database have cited the publication being examined. In addition to the citation numbers of an individual publication, citation databases typically allow users to review things such as how many times a certain author or the publications of a certain organisation have been cited. The number of citations received by a publication varies in different databases. Citation databases usually focus on scientific journal articles, but they can also include monographs, conference publications and reports. No citation database includes all publications. It should also be kept in mind that certain fields of sciences (such as medical and natural sciences) have better representation in databases than others (such as social sciences, humanities and arts). The number of citations in databases also depends on the length of the period from which the database has citation data and on how often the citation data is updated in the database. All databases also have some errors. The content of databases is constantly changing, because they add new materials to their collection and remove outdated content while also updating citation data of older publications.

The two of the most well-known multidisciplinary databases are Web of Science (WoS) by Clarivate Analytics and Scopus by Elsevier. Both WoS and Scopus contain curated materials and they have certain quality criteria for their database content, for example, the journals must apply a peer review process and have an editorial board and an ISSN identifier. The title and abstract of the article must be written in English. Citation data can also be analysed in the publicly available Dimensions and Google Scholar (GS) services. Both Dimensions and GS index materials into their collections with automated methods, without a separate validation process. The basic content and functionalities of the Dimensions database are available openly and free of charge. Paying subscribers also have access to a version with more extensive content and more versatile analysis functionalities. GS is a free search engine that is specialised in finding scientific information, but it also holds non-scientific materials such as thesis works. Its citation data also includes many citations from non-scientific publications, and their number of citations differs greatly from that of the WoS and Scopus databases. This guide presents GS along with citation databases, although it is not an actual database. All three databases, as well as Google Scholar, have been presented in more detail in their own chapters.

Comparing the sources of citation data

Because the indexing methods and collection policy strategies of materials in citation databases vary significantly, so does the content of these databases. WoS and Scopus both apply a selective collection policy and have editorial boards with experts of different fields that help them select and index their content. Dimensions is heavily reliant on machine learning and data processing implemented with automated algorithms instead of manual curating done by experts. Dimensions mainly indexes all scientific publications and data sets with a DOI identifier. The automated methods of Google Scholar, on the other hand, crawl the internet and index all publications that they recognise as having an academic structure.

The publishing practices of different disciplines, such as the publication type and language, are reflected in the coverage of the databases. The traditional citation databases, such as WoS and Scopus, mainly contain scientific journals. Fields of sciences where books are a typical type of publication are poorly represented in these databases, in particular. The content is also often lacking in terms of conference publications. Typically, social sciences and humanities have the poorest coverage. The fact that the databases mainly focus on literature written in English is another limiting factor of their coverage. Although the databases also accept publications in languages other than English, one of the selection criteria for the articles published in these publication channels is that their abstracts and titles are written in English. Dimensions and GS, which are based on automated indexing methods, reach a significantly higher degree of coverage in most fields of science. Dimensions is noticeably more extensive than WoS or Scopus, especially when it comes to edited books, their chapters or monographs.

Several studies have been carried out on the differences between databases and on the extent of their coverage of materials from different fields of science. Many of these studies show that the coverage of the WoS Core Collection database is lower within many fields of science than that of its competing databases (e.g. Mongeon, P. & Paul-Hus, A. 2016; Martín-Martín, A. et al. 2021; Singh, V.K. et al. 2021; Visser, M. et al. 2021). The citation data source that proved in many studies to have the best coverage in nearly all fields of science out of all sources presented in this guide is Google Scholar (e.g. Harzing, A.-W. 2019; Martín-Martín, A. et al. 2021). Some recent studies about the coverage of citation database materials have been listed at the end of this chapter. Additionally, a coverage comparison of databases carried out on the basis of publications produced by the University of Jyväskylä can be found as an attachment

When selecting a database, however, the coverage of its materials is not the only criteria. The search functions and the quality of the indexed materials also matter. Even though GS has greater coverage than its competitors, its search functions have some limitations, such as limited use of Boolean operators and very limited filtering options for search results. The traditional WoS and Scopus databases that have been on the market for a long time have weaker coverage, the materials are indexed into the databases at a slower pace, and they are also paid services. On the other hand, they both have a wide range of search functions, and the materials indexed by them have been validated. The table 1 presents the key functionalities and content of the Web of Science, Scopus and Dimensions databases as well as the Google Scholar service to allow for easy comparisons of the citation databases. In the name of responsible publication metrics, the citation database used for an analysis should be selected, in addition to the usage requirements, by considering the needs of the scientific field being analysed. If possible, it is recommended that more than one citation database be used for any publication-based analyses.