Google Scholar, Scopus and Web of Science

Another comparison of these three sources by Kathleen Bauer and Nisa Bakkalbasi from Yale in their recent article published in D-Lib Magazine

In the article they are also questioning Google Scholar's content and validity as a true scholarly resource:

"However, it is important for all researchers to note that until Google Scholar gives a full account of what material it is indexing and how often that index is updated, it cannot be considered a true scholarly resource in the sense that Web of Science and Scopus are. An understanding of the material being covered is central to the validity of any search of scholarly material."