Including cited non-source items in a large-scale map of science: What difference does it make?

Cited non-source documents such as articles from regional journals, conference papers, books and book chapters, working papers and reports have begun to attract more attention in the literature. Most of this attention has been directed at understanding the effects of including non-source items in research evaluation. In contrast, little work has been done to examine the effects of including non-source items on science maps and on the structure of science as reflected by those maps. In this study we compare two direct citation maps of a 16-year set of Scopus documents - one that includes only source documents, and one that includes non-source documents along with the source documents. In addition to more than doubling the contents of the map, from 19 M to 43 M documents, the inclusion of non-source items strongly augments the social sciences relative to the natural sciences and medicine and makes their position in the map more central. Books are also found to play a significant role in the map, and are much more highly cited on average than articles.