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Gunther Eysenbach, Citation advantage of open access articles, PLoS Biology, May 2006.
Abstract: Open access (OA) to the research literature has the potential to accelerate recognition and dissemination of research findings, but its actual effects are controversial. This was a longitudinal bibliometric analysis of a cohort of OA and non-OA articles published between June 8, 2004, and December 20, 2004, in the same journal (PNAS: Proceedings of the National Academy of Sciences). Article characteristics were extracted, and citation data were compared between the two groups at three different points in time: at "quasi-baseline" (December 2004, 0-6 mo after publication), in April 2005 (4-10 mo after publication), and in October 2005 (10-16 mo after publication). Potentially confounding variables, including number of authors, authors' lifetime publication count and impact, submission track, country of corresponding author, funding organization, and discipline, were adjusted for in logistic and linear multiple regression models. A total of 1,492 original research articles were analyzed: 212 (14.2% of all articles) were OA articles paid by the author, and 1,280 (85.8%) were non-OA articles. In April 2005 (mean 206 d after publication), 627 (49.0%) of the non-OA articles versus 78 (36.8%) of the OA articles were not cited (relative risk = 1.3 [95% Confidence Interval: 1.1-1.6]; p = 0.001). 6 mo later (mean 288 d after publication), non-OA articles were still more likely to be uncited (non-OA: 172 [13.6%], OA: 11 [5.2%]; relative risk = 2.6 [1.4-4.7]; p < 0.001). The average number of citations of OA articles was higher compared to non-OA articles (April 2005: 1.5 [SD = 2.5] versus 1.2 [SD = 2.0]; Z = 3.123; p = 0.002; October 2005: 6.4 [SD = 10.4] versus 4.5 [SD = 4.9]; Z = 4.058; p < 0.001). In a logistic regression model, controlling for potential confounders, OA articles compared to non-OA articles remained twice as likely to be cited (odds ratio = 2.1 [1.5-2.9]) in the first 4-10 mo after publication (April 2005), with the odds ratio increasing to 2.9 (1.5-5.5) 10-16 mo after publication (October 2005). Articles published as an immediate OA article on the journal site have higher impact than self-archived or otherwise openly accessible OA articles. We found strong evidence that, even in a journal that is widely available in research libraries, OA articles are more immediately recognized and cited by peers than non-OA articles published in the same journal. OA is likely to benefit science by accelerating dissemination and uptake of research findings.
The same issue contains an editorial by Catriona J. MacCallum and Hemai Parthasarathy, Open access increases citation rate. Excerpt:
In the current study, Eysenbach compared citations compiled by Thomson Scientific (formerly Thomson ISI) to individual articles published between June 2004 and December 2004 in the same journal --namely, Proceedings of the National Academy of Sciences (PNAS), which announced its open-access option for authors on June 8 of that year, with an associated publication charge of US$1,000. Non-OA articles in PNAS are subject to a six-month “toll-access” delay before the article becomes publicly available. The results of this natural experiment are clear: in the 4 to 16 months following publication, OA articles gained a significant citation advantage over non- OA articles during the same period. They are twice as likely to be cited 4 to 10 months after publication and almost three times as likely between 10 and 16 months. Given that PNAS delays open access for only six months, the disparity between OA and non-OA articles in journals where the delay is longer or where articles remain “toll-access” is likely to be even greater....PNAS was one of the first journals to offer an open-access option to its authors. However, such hybrid journals are increasing: Blackwell, Springer, and Oxford University Press now provide this option as well. This means that similar experiments can be replicated. Moreover, although the evidence from the current analysis argues most strongly for a time advantage in citation for OA articles, a study over longer periods will reveal whether this translates into a sustained increase in the number of citations. In the meantime, open-access advocates should be emboldened by tangible evidence for what has seemed obvious all along.
At the same time, Eysenbach has published a companion editorial, The Open Access Advantage, in his own journal, the Journal of Medical Internet Research.
Abstract: A study published today in PLoS Biology provides robust evidence that open-access articles are more immediately recognized and cited than non-OA articles. This editorial provides some additional follow up data from the most recent analysis of the same cohort in April 2006, 17 to 21 months after publication. These data suggest that the citation gap between open access and non-open access papers continues to widen. I conclude with the observation that the “open access advantage” has at least three components: (1) a citation count advantage (as a metric for knowledge uptake within the scientific community), (2) an end user uptake advantage, and (3) a cross-discipline fertilization advantage. More research is needed, and JMIR is inviting research on all aspects of open access. As the advantages for publishing open access from a researchers' point of view become increasingly clear, questions around the sustainability of open access journals remain. This journal is a living example that "lean publishing" models can create successful open access journals. Open source tools which have been developed by the Public Knowledge Project at the University of British Columbia with contributions from the Epublishing & Open Access group at the Centre for Global eHealth Innovation in Toronto are an alternative to hosting journals on commercial open access publisher sites.