When it comes to searching for scientific literature, Google Scholar has become a go-to resource for a growing number of researchers. The powerful academic search engine seems to comb through every academic study in existence. But figuring out exactly how many papers are covered by Google Scholar isn’t easy, recent research shows—in part because of the company’s secretive, tightlipped nature. And some scholars warn the service may be inflating citation counts, although that may not necessarily be a bad thing.
Figuring out how many documents are indexed in traditional bibliographic databases, such as Thomson Reuters’s Web of Science and Elsevier’s Scopus, is a piece of cake—a simple query is all it takes. Microsoft Academic Search is similarly transparent. Google Scholar, however, offers no such tools to bibliometric researchers, and the Web search giant has declined to publish the information.
To come up with a tally, bibliometricist Enrique Orduña-Malea of the Polytechnic University of Valencia in Spain and his colleagues used four different methods to estimate Google Scholar’s total number of documents. Although each method has distinct limitations, all but one yield similar results, the researchers report in a study posted to the arXiv preprint server earlier this year and updated this month. The number: 160 million indexed documents (plus or minus 10%), including journal articles, books, case law, and patents.