Hannah Noll wrote her Master's thesis comparing coverage of art history in Google Scholar with Arts and Humanities Citation Index, Bibliography of the History of Art, and Art Index/Art Retrospective Index. Noll is a graduate graduate student in the UNC School of Library and Information Science. The subscription databases generally retrieved more citations while Google Scholar was more consistent in coverage.
Here is the abstract quoted from the thesis:
"This study evaluates the content coverage of Google Scholar and three commercial databases (Arts & Humanities Citation Index, Bibliography of the History of Art and Art Full Text/Art Index Retrospective) on the subject of art history. Each database is tested using a bibliography method and evaluated based on Péter Jacsó’s scope criteria for online databases. Of the 472 articles tested, Google Scholar indexed the smallest number of citations (35%), outshone by the Arts & Humanities Citation Index which covered 73% of the test set. This content evaluation also examines specific aspects of coverage to find that in comparison to the other databases, Google Scholar provides consistent coverage over the time range tested (1975-2008) and considerable access to article abstracts (56%). Google Scholar failed, however, to fully index the most frequently cited art periodical in the test set, the Artforum International. Finally, Google Scholar’s total citation count is inflated by a significant percentage (23%) of articles which include duplicate, triplicate or multiple versions of the same record."