Comparison of three options for citation tracking: Google Scholar, Scopus and Web of Science

There are some very interesting results in this study published in the June 29, 2006 issue of "Biomedical Digital Libraries".
The authors suggest that "Our data indicate that the question of which tool provides the most complete set of citing literature may depend on the subject and publication year of a given article".

You can read this article at:
http://www.bio-diglib.com/content/3/1/7