Open Access (OA) is a model for publishing scholarly peer reviewed journals, made possible by the Internet. The full text of OA journals and articles can be freely read, as the publishing is funded through means other than subscriptions. Empirical research concerning the quantitative development of OA publishing has so far consisted of scattered individual studies providing brief snapshots, using varying methods and data sources. This study adopts a systematic method for studying the development of OA journals from their beginnings in the early 1990s until 2009. Because no comprehensive index of OA articles exists, systematic manual data collection from journal web sites was conducted based on journal-level data extracted from the Directory of Open Access Journals (DOAJ). Due to the high number of journals registered in the DOAJ, almost 5000 at the time of the study, stratified random sampling was used. A separate sample of verified early pioneer OA journals was also studied. The results show a very rapid growth of OA publishing during the period 1993–2009. During the last year an estimated 191 000 articles were published in 4769 journals. Since the year 2000, the average annual growth rate has been 18% for the number of journals and 30% for the number of articles. This can be contrasted to the reported 3,5% yearly volume increase in journal publishing in general. In 2009 the share of articles in OA journals, of all peer reviewed journal articles, reached 7,7%. Overall, the results document a rapid growth in OA journal publishing over the last fifteen years. Based on the sampling results and qualitative data a division into three distinct periods is suggested: The Pioneering years (1993–1999), the Innovation years (2000–2004), and the Consolidation years (2005–2009).

BackgroundThe Internet has recently made possible the free global availability of scientific journal articles. Open Access (OA) can occur either via OA scientific journals, or via authors posting manuscripts of articles published in subscription journals in open web repositories. So far there have been few systematic studies showing how big the extent of OA is, in particular studies covering all fields of science.Methodology/Principal FindingsThe proportion of peer reviewed scholarly journal articles, which are available openly in full text on the web, was studied using a random sample of 1837 titles and a web search engine. Of articles published in 2008, 8,5% were freely available at the publishers' sites. For an additional 11,9% free manuscript versions could be found using search engines, making the overall OA percentage 20,4%. Chemistry (13%) had the lowest overall share of OA, Earth Sciences (33%) the highest. In medicine, biochemistry and chemistry publishing in OA journals was more common. In all other fields author-posted manuscript copies dominated the picture.Conclusions/SignificanceThe results show that OA already has a significant positive impact on the availability of the scientific journal literature and that there are big differences between scientific disciplines in the uptake. Due to the lack of awareness of OA-publishing among scientists in most fields outside physics, the results should be of general interest to all scholars. The results should also interest academic publishers, who need to take into account OA in their business strategies and copyright policies, as well as research funders, who like the NIH are starting to require OA availability of results from research projects they fund. The method and search tools developed also offer a good basis for more in-depth studies as well as longitudinal studies.

Open Access (OA) is the free unrestricted access to electronic versions of scholarly publications. For peer reviewed journal articles there are two main routes to OA, publishing in OA journals (gold OA) or archiving of article copies or manuscripts at other web locations (green OA). This study focuses on summarizing and extending upon current knowledge about green OA. A synthesis of previous studies indicates that the green OA coverage of all published journal articles is approximately 12 %, with substantial disciplinary variation. Typically, green OA copies become available with considerable time delays, partly caused by publisher imposed embargo periods, and partly by author tendencies to archive manuscripts only periodically. Although green OA copies should ideally be archived in proper repositories, a large share is stored on home pages and similar locations, with no assurance of long-term preservation. Often such locations contain exact copies of published articles, which may infringe on the publisher's exclusive rights. The technical foundation for green OA uploading is becoming increasingly solid, which is largely due to the rapid increase in the number of institutional repositories. The number of articles within the scope of OA mandates, which strongly influence the selfarchival rate of articles, is nevertheless still low.3

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