It is also a question that has been written quite a bit about. In the previous blog post you could read that JSTOR has over 150 million attempts yearly to download articles that the users do not have access to. There are studies which indicate that open access articles get more citations directly after publication. There are also studies which indicate that there is no big difference between the number of citations between open access articles and toll access articles. It takes some years for a journal to build up its reputation which has made these studies difficult to do. Results in an article from 2006 shows that open access articles are cited quicker after publication. The study shows that a certain period after publication (approx. 200 days) 49 % of toll access articles and 37 % of open access articles had not gotten citations yet. The study also showed that open access articles were used more although researchers had access to the toll access journals through a library. Also, it showed that open access articles which were published in same journals as non-open access articles, received more citations.
Yet another article was published in 2008. It showed that open access articles had on average nine citations while toll access articles had received five. Also, this study shows that there is a difference between disciplines, e.g. sociology had the biggest advantage but the least amount of open access articles.
Even more studies can be found at The Open Citation Project home page.
There are those who mean that it is difficult to prove citation advantage because there is a problem with the methods and definitions which have been used in these studies, i.e. that open access is defined as freely available material and not as publishers’ article processing charge business model. Criticism against the methods have to do with control groups, samples and which conclusions can be drawn. These problems are discussed in Dr Henk Moeds article “Does open access publishing increase citation or download rates”.
There is research which indicates that open access publishing increases citations but there is also studies which indicate the opposite. There are also studies which indicate that making research data available increases citations. At least initially it is better to publish open access because these articles are disseminated quicker. Open access journals have matured now and also, there are other indicators for dissemination and impact due to the development of altmetrics.
 Norris, M., Oppenheim, C. & Rowland, F. (2008). The citation advantage of open-access articles. Journal of the American Society for Information Science and Technology. 59(12), ss. 1867-2038. doi: 10.1002/asi.20898
 Piwowar, H.A., Day, R.S. & Fridsma, D.B. (2007). Sharing Detailed Research Data Is Associated with Increased Citation Rate. PLoS ONE 2(3): e308. doi:10.1371/journal.pone.0000308