The research literature published by information science researchers shows that data extracted from the major citation search engines, and especially from any single source, is likely to be significantly under-represented for Social Science and especially Humanities research owing to discipline-specific practices of scholarly communication that value monograph, conference proceedings, local journals and dissemination in languages other than English, and which are very different from practices used in most of the sciences. In simple terms, the citation indices do not contain information on all the research outputs from these communities, rendering attempts to generate reliable metrics of impact or to assess the research performance of institutions, departments or individual scholars in the Social Sciences and especially the Humanities highly problematic if using citation index data alone.
However, the existence of such structural limitations on the data that can be collected from WoS (and Scopus to a similar extent), does not necessarily mean that discipline maps based on such data suffer from the same limitations to the same degree. The extent to which documents are absent in the maps depends on the extent to which the collated citing documents (as the start of a citation link) make reference to other cited documents (as the end of a citation link) that are not included in the citation indices, not published in English, or not published online. Bibliographic maps are also shaped by the sources cited in archaeological research outputs, not solely by the features of the inputted document set. For the same input limitations into cited documents to continue, different forms of disciplinary citation practices would need to be followed for international journals, national journals, online journals, books, conference proceedings and so forth. There is no published research to suggest that this is the case. In the maps, for example, there are monographs, there are publications in French, German, Spanish, Portugese and a few other languages, and there are online journals such as Internet Archaeology and Before Farming included as cited documents. In the future it is possible that maps might also include documents from the grey literature if they are cited by indexed publications. Therefore, while the set of input documents on which these maps are based has recognisable limitations as described above, the maps included here, based on the citations from the input set to other documents, have no such necessary limitations.
Finally, it is important to note that WoS recognises some of the problems described above and has started an Emerging Sources Citation Index to increase its coverage of other journals and languages, as well as a book citation index; with time these should help alleviate some of the limitations of the flagship citation indices.
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