BackgroundArticles whose authors have supplemented subscription-based access to the publisher's version by self-archiving their own final draft to make it accessible free for all on the web (“Open Access”, OA) are cited significantly more than articles in the same journal and year that have not been made OA. Some have suggested that this “OA Advantage” may not be causal but just a self-selection bias, because authors preferentially make higher-quality articles OA. To test this we compared self-selective self-archiving with mandatory self-archiving for a sample of 27,197 articles published 2002–2006 in 1,984 journals.Methdology/Principal FindingsThe OA Advantage proved just as high for both. Logistic regression analysis showed that the advantage is independent of other correlates of citations (article age; journal impact factor; number of co-authors, references or pages; field; article type; or country) and highest for the most highly cited articles. The OA Advantage is real, independent and causal, but skewed. Its size is indeed correlated with quality, just as citations themselves are (the top 20% of articles receive about 80% of all citations).Conclusions/SignificanceThe OA advantage is greater for the more citable articles, not because of a quality bias from authors self-selecting what to make OA, but because of a quality advantage, from users self-selecting what to use and cite, freed by OA from the constraints of selective accessibility to subscribers only. It is hoped that these findings will help motivate the adoption of OA self-archiving mandates by universities, research institutions and research funders.

Abstract. The research access/impact problem arises because journal articles are not accessible to all of their would-be users, hence they are losing potential research impact. The solution is to make all articles Open Access (OA, i.e., accessible online, free for all). OA articles have significantly higher citation impact than non-OA articles. There are two roads to OA: the "golden" road (publish your article in an OA journal) and the "green" road (publish your article in a non-OA journal but also self-archive it in an OA archive). About 10% of journals are gold, but over 90% are already green (i.e., they have given their authors the green light to self-archive); yet only about 10-20% of articles have been self-archived. To reach 100% OA, self-archiving needs to be mandated by researchers' employers and funders, as they are now increasingly beginning to do.The research journal-affordability problem and the resulting university libraries' journal budget crisis were what first brought the research article-access/impact problem to light, but the journal-affordability problem and the articleaccess/impact problem are not the same. According to Ulrichsweb [http://www.ulrichsweb.com/ulrichsweb/analysis/], about 25,000 peer-reviewed research journals exist worldwide, across all disciplines and languages, publishing about 2.5 million articles per year. But because journal prices keep rising and library budgets are limited, each university can afford only a small portion of that total. That means their users have access to only a fraction of those articles, even though, in the online age, we would have expected otherwise. This is the research journal-affordability problem.What the journal-affordability problem unmasked was a further problem: As a consequence of the fact that most of their would-be users at most universities cannot access most of the 2.5 million articles published yearly (because their universities cannot afford the journal access-tolls), a significant portion of the potential research impact of those inaccessible articles is being lost. An article's research impact is the degree to which its findings are read, used, applied, built-upon and cited by researchers in their own further research and applications. Research impact is a measure of the progress and productivity of research. That is the reason why researchers' careers (their salaries, promotions, tenure, funding, prestige, prizes) depend on their impact; it is also why their universities (which co-benefit from the research funding, progress and prestige) as well as their research funding agencies (which are answerable for the way they spend tax-payers' money) reward research impact.Merely to do the research and then put your findings in a desk-drawer is no better than not doing the research at all. Researchers must submit their research to peer review (Harnad 1998) and then "publish or perish," so others can use and apply their findings. But getting findings peer-reviewed and published is not enough either: Other researchers must find the findings ...

Abstract. The research access/impact problem arises because journal articles are not accessible to all of their would-be users, hence they are losing potential research impact. The solution is to make all articles Open Access (OA, i.e., accessible online, free for all). OA articles have significantly higher citation impact than non-OA articles. There are two roads to OA: the "golden" road (publish your article in an OA journal) and the "green" road (publish your article in a non-OA journal but also self-archive it in an OA archive). About 10% of journals are gold, but over 90% are already green (i.e., they have given their authors the green light to self-archive); yet only about 10-20% of articles have been self-archived. To reach 100% OA, self-archiving needs to be mandated by researchers' employers and funders, as they are now increasingly beginning to do.The research journal-affordability problem and the resulting university libraries' journal budget crisis were what first brought the research article-access/impact problem to light, but the journal-affordability problem and the articleaccess/impact problem are not the same. According to Ulrichsweb [http://www.ulrichsweb.com/ulrichsweb/analysis/], about 25,000 peer-reviewed research journals exist worldwide, across all disciplines and languages, publishing about 2.5 million articles per year. But because journal prices keep rising and library budgets are limited, each university can afford only a small portion of that total. That means their users have access to only a fraction of those articles, even though, in the online age, we would have expected otherwise. This is the research journal-affordability problem.What the journal-affordability problem unmasked was a further problem: As a consequence of the fact that most of their would-be users at most universities cannot access most of the 2.5 million articles published yearly (because their universities cannot afford the journal access-tolls), a significant portion of the potential research impact of those inaccessible articles is being lost. An article's research impact is the degree to which its findings are read, used, applied, built-upon and cited by researchers in their own further research and applications. Research impact is a measure of the progress and productivity of research. That is the reason why researchers' careers (their salaries, promotions, tenure, funding, prestige, prizes) depend on their impact; it is also why their universities (which co-benefit from the research funding, progress and prestige) as well as their research funding agencies (which are answerable for the way they spend tax-payers' money) reward research impact.Merely to do the research and then put your findings in a desk-drawer is no better than not doing the research at all. Researchers must submit their research to peer review (Harnad 1998) and then "publish or perish," so others can use and apply their findings. But getting findings peer-reviewed and published is not enough either: Other researchers must find the findings ...