Swan, A. (2010) The Open Access citation advantage: Studies and results to date. Technical Report. School of Electronics & Computer Science, University of Southampton.
Abstract: This paper presents a summary of reported studies on the Open Access citation advantage. There is a brief introduction to the main issues involved in carrying out such studies, both methodological and interpretive. The study listing provides some details of the coverage, methodological approach and main conclusions of each study.
Added Mar 15 2010
See also (thanks to Peter Suber for spotting this study!):
Wagner, A. Ben (2010) Open Access Citation Advantage: An Annotated Bibliography. Issues in Science and Technology Librarianship. 60. Winter 2010
“Far more issues about OA and meta analysis have been raised in this thread for me to [be able to] comment on. But having dedicated 35 years of my efforts to meta analysis and 20 to OA, I can?t resist a couple of quick observations.
Holding up one set of methods (be they RCT or whatever) as the gold standard is inconsistent with decades of empirical work in meta analysis that shows that ?perfect studies? and ?less than perfect studies? seldom show important differences in results. If the question at hand concerns experimental intervention, then random assignment to groups may well be inferior as a matching technique to even an ex post facto matching of groups. Randomization is not the royal road to equivalence of groups; it?s the road to probability statements about differences.
Claims about the superiority of certain methods are empirical claims. They are not a priori dicta about what evidence can and can not be looked at.”
Glass, G.V.; McGaw, B.; & Smith, M.L. (1981). Meta-analysis in Social Research. Beverly Hills, CA: SAGE.
Rudner, Lawrence, Gene V Glass, David L. Evartt, and Patrick J. Emery (2000). A user’s guide to the meta-analysis of research studies. ERIC Clearinghouse on Assessment and Evaluation, University of Maryland, College Park.