Martin Boeker, Werner Vach and Edith Motschall have published "Google Scholar as Replacement for Systematic Literature Searches: Good Relative Recall and Precision Are Not Enough" in BMC Medical Research Methodology.
Here's an excerpt:
The objectives of this work are to measure the relative recall and precision of searches with Google Scholar under conditions which are derived from structured search procedures conventional in scientific literature retrieval; and to provide an overview of current advantages and disadvantages of the Google Scholar search interface in scientific literature retrieval. . . .
The reported relative recall must be interpreted with care. It is a quality indicator of Google Scholar confined to an experimental setting which is unavailable in systematic retrieval due to the severe limitations of the Google Scholar search interface. Currently, Google Scholar does not provide necessary elements for systematic scientific literature retrieval such as tools for incremental query optimization, export of a large number of references, a visual search builder or a history function. Google Scholar is not ready as a professional searching tool for tasks where structured retrieval methodology is necessary.
Digital Scholarship | Digital Scholarship Publications Overview | Sitemap