Because he did all this work so I don’t have to, Ben Wagner “…review[ed] studies on the citation advantage (or in a few studies, non-advantage) that open access articles have over non-OA articles. http://www.buffalo.edu/~abwagner/OA-CiteImpact-Bibliography.doc.
I normally would not note this update, excepting a colleague in Great Britain call my attention to an amazing, massive October 2014 study of OA articles done for the European Commission, an analysis of over 1 million articles indexed in Scopus<http://www.elsevier.com/__data/assets/pdf_file/0007/148714/scopus_facts_and_figures.pdf> from 1996-2013. Data on the growth of OA, the proportion of various types of OA articles, and the OCA citation advantage are reported. It is the only study to my knowledge that includes a breakdown of OA articles by country, region, discipline, publication year, and type of OA: gold, green (in official repository), other (free, but not in official repository.
Since this is not a burning issue for everyone on this list, please refer to my blog post <http://libweb.lib.buffalo.edu/blog/sciences/?p=962> or the bibliography cited above for a slightly expanded summary. The report’s 7-page executive summary is well worth a read: http://science-metrix.com/files/science-metrix/publications/d_1.8_sm_ec_dg-rtd_proportion_oa_1996-2013_v11p.pdf.
–A. Ben Wagner, Sciences Librarian
Science & Engineering Information Center
226 Capen Hall (Silverman Library)
University at Buffalo
I read the summary and would recommend you do the same.