A number of studies have now been carried out on the effect of Open Access on citations to articles, showing the increased citation impact that Open Access can bring. Steve Lawrence's was the earliest study, finding in 2001 that free online access tripled citations of computer science papers. Michael Kurtz's early study on the astronomy literature demonstrated that Open Access can double the readership of articles. A study published in the same year by Brody & Harnad showed an increase in citations to articles in several disciplines as a result of their being Open Access. And Kristin Antelman's work corroborated these findings in the fields of philosophy, political science, electrical & electronic engineering and mathematics. More recently, Michael Norris has published his doctoral thesis that reports similar impact advantage in the form of increased citations for articles in four other disciplines - economics, applied mathematics, sociology and ecology. There is a summary of all the impact studies to date here.
The results from Stevan Harnad's research groups in Southampton and Montreal are shown graphically below.
The methodology used was to crawl the web looking for scholarly articles that are available in full-text on an Open Access basis. Once one was located, other articles from the same issue of the same journal were sought with which to compare it. This methodology ensures that articles are matched as closely as possible in all respects - journal, issue, research area, audience and so on - except that one is Open Access and the other is Toll Access; that is, it is only available to subscribers of the journal. (Open Access journals in which all articles are freely accessible were of course systematically excluded from the study.) Citations to such paired articles were compared and measured. The aggregated results for all the scholarly disciplines studied so far show that for every discipline there is an increase in citations when articles are Open Access compared to their non-open Access (Toll Access) partners.
The size of the increase does vary (year to year and field to field), as also found by the other studies cited above. The reasons for that are not fully clear but we do know that the citation increase due to Open Access - the so-called 'Open Access advantage' - is a complex advantage composed of several elements. These are:
- Download (or Usage) Advantage (DA): OA articles are downloaded significantly more, and this early DA has also been shown to be predictive of a later citation advantage in physics (Brody et al. 2006)
- Competitive Advantage (CA): OA articles are in competition with non-OA articles, and to the extent that OA articles are relatively more accessible than non-OA articles, they can be used and cited more
- Accessibility Advantage (AA) originating from those users whose institutions do not have subscription access to the journal in which the article appeared. All articles can potentially benefit from AA, which (unlike CA) persists even after universal OA is reached as all articles continue to enjoy AA's full benefit
- Quality Advantage (QA), which is often (erroneously) taken to be the same as quality bias. QA is the citation advantage that high-quality articles have over articles of poorer quality
- Quality Bias (QB), which is the tendency for better authors to self-archive better articles if they are in a position to be selective (i.e. not under a mandate to make all their outputs Open Access)
- Early Access Advantage (EAA), which is the citation advantage enjoyed by articles made Open Access earlier - even at the unrefereed preprint stage
More detail on this topic can be found in an article by Harnad (2008) here. Not all these advantages will persist when the entire literature is Open Access, of course. If everything is Open Access, how can there be any advantages? Well, the quality bias (QB) and the competitive advantage (CA) will certainly be temporary and will disappear once all the literature is Open Access. But AA will continue to benefit all articles overall, and QA will particularly benefit the articles of higher quality.
The Early Access Advantage (EAA) and the Download Advantage (DA) will likewise persist inasmuch as not all articles are self-archived (made Open Access) equally early. The longer it takes for an author to self-archive a paper (even a pre-refereeing preprint), the more that paper is missing out on these elements of the Open Access advantage.
Hence the EAA and DA, in contrast, will continue to contribute to the OA advantage even after universal OA is reached, when all postprints are being made OA immediately upon publication, compared to pre-OA days (as Kurtz has shown for Astronomy, which has already reached universal post-publication OA). Kurtz's work on articles in the Astronomy Data Service database (which is Open Access) shows that Early Access Advantage is both an important and persistent phenomenon, even within an entirely Open Access corpus of literature.
So while it is predicted that the Open Access citation advantage will subside as the proportion of literature that is Open Access increases, it is not expected to entirely disappear. A citation advantage will continue to accrue to articles that are openly disseminated early in the publication process.
See also an account of how Open Access has increased the citations of one scientist.