Here's an evidence summary from the newest issue of one of my favorite journals, Evidence-Based Library & Information Practice. Evidence summaries are important as the first efforts (of which I am aware) to systematically perform critical evaluations of our own literature. This particular work hits on questions I have had - I have found Google Scholar to be an important way in which I can discover research literature across disciplines - it's pretty much the only game in town, federated search engines notwithstanding.
Walters, W. H. (2009). Google Scholar search performance: Comparative recall and precision. portal:
Libraries and the Academy, 9(1), 5-24.
from Evidence Based Library and Information Practice
"Objective – To compare the search performance (i.e., recall and precision) of Google Scholar with that of 11 other bibliographic databases when using a keyword search to find references on later-life migration.
Main Results – Google Scholar and AgeLine obtained the largest number of results (20,400 and 311 hits respectively) for the keyword search, elderly and migration. Database performance was evaluated with regards to the recall and precision of its search results. Google Scholar and AgeLine also obtained the largest total number of relevant search results out of all the potential results that could be obtained on later-life migration (41/155 and 35/155 respectively). No individual database produced the highest recall for every set of search results listed, i.e., for the first 10 hits, the first 20 hits, etc. However, Google Scholar was always in the top four databases regardless of the number of search results displayed. Its recall rate was consistently higher than all the other databases when over 56 search results were examined, while Medline out-performed the others within the first set of 50 results.
Instructors and librarians may wish to support the use of databases that increase students’ contact with high-quality research documents (i.e.., documents that are authoritative, well written, contain a strong analysis, or demonstrate quality in other ways). The study’s findings indicate that Google Scholar is an example of one such database, since it obtained a large number of references to the relevant papers on the topic searched."