This study confirms that the OA advantage is a statistically significant, independent positive increase in citations, even when we control the independent contributions of many other salient variables (article age, journal impact factor, number of authors, number of pages, number of references cited, Review, Science, USA author). All these other variables are of course correlated with citation counts, so the fact that OA continues to correlate significantly with an independent positive increase in citation counts even when the contributions of all these other correlates are calculated independently means that the OA Advantage is not just a bias arising from either a random or a systematic imbalance in the other correlates of citations.
Moreover, the OA advantage is just as great when the OA is mandated (with mandate compliance rate ~60%) as when it is self-selective (self-selection rate ~15%). This makes it highly unlikely that the OA advantage is either entirely or mostly the result of an author bias toward selectively self-archiving higher quality – hence higher citability – articles. Nor are the main effects the result of institutional citation advantages, as the institutions were among the independent predictor variables in the logistic regression; the outcome pattern and significance is also unaltered by removing CERN, the only one of the four institutions that might conceivably have biased the outcome because its papers were all in one field and tended to be of higher quality, hence higher citability overall.
Since, with the exception of our one unidisciplinary institute – CERN (high energy physics) – the pluridisciplinary articles from the three other mandated institutional repositories are mostly not in fields that habitually self-archive their unrefereed preprints well before publication (as many in high energy physics do), nor in fields that already have effective OA for their published postprints (as astronomy does: 
, ), it is also unlikely that the OA advantage is either entirely or mostly just an early-access (prepublication) advantage 
. This will eventually be testable once there are enough reliable data available on deposit-date, relative to publication-date, for a large enough body of self-archived OA articles. In any case, an early-access advantage in a preprint self-archiving field translates into a generic postpublication OA advantage in that vast majority of fields in which authors do not self-archive their prepublication preprints and so their published postprints are accessible only to subscribers – except if they have also been self-archived. The OA mandates all apply only to refereed postprints, self-archived upon publication, not to pre-refereeing preprints, self-archived before publication.
This study confirms that the OA advantage is substantially greater for articles that have successfully met the quality standards of higher-impact journals and it is also greater in the higher-citation ranges for individual papers within each journal-impact level. The typical Pareto distribution for citations whereby the top 10–20% of articles receive about 80–90% of all citations 
, is present in our own sample of 708,219 articles extracted from Thomson-Reuters from 1998 to 2007: about 20% of articles
received about 80% of all citations. In addition, 10% of journals
receive 90% of all citations.
The implication is that OA itself will not make an unusable (hence uncitable) paper more used and cited (although the proportion of uncited papers has been diminishing with time; 
). But wherever there are subscription-based constraints on accessibility, providing OA will increase the usage and citation of the more usable and citable papers, probably in proportion to their importance and quality, hence citability. We accordingly infer from our results that the most likely cause of the OA citation advantage is not author self-selection
toward making more citable articles OA, but user self-selection
toward using and citing the more citable articles – once OA self-archiving has made them accessible to all users, rather than just to those whose institutions could afford subscription access. In other words, we conclude that the OA advantage is a quality advantage
, rather than a quality bias
: it is not that the higher quality articles – the ones that are more likely to be selectively cited anyway – are more likely to be made OA self-selectively by their authors, but that the higher quality articles that are more likely to be selectively cited are made more accessible, hence more citable, by being made OA.
Our results also suggest the possibility that mandated OA might have some further independent citation advantage of its own, over self-selected OA – but until and unless this effect is replicated, it is more likely that this small, previously unreported effect was due to chance or sampling error. If there does indeed prove to be an independent “mandate advantage” over and above OA itself, a possible interpretation would be the reverse of the self-selection hypothesis: There may be a higher proportion of higher-quality work among the 80% that are not being made OA on a self-selective basis today than among the 20% that are; the result is that the OA mandates serve to help bring this “cream of science” to the top.
It also needs to be noted that some of the factors contributing to the OA advantage are permanent, whereas others will shrink as OA rises from its current 15–20% level and will disappear completely at 100% OA. All competitive advantage of OA over non-OA (because OA is more accessible) will of course vanish at 100% OA (as will the possibility of concurrent measurement of the OA Advantage). Any self-selective bias (whether positive or negative) will likewise disappear at 100% OA. What will remain will be the quality advantage itself (the tendency of researchers to selectively use and cite the best research, if they can access it), but maximized by leveling the playing field, making everything accessible to every user online.
There will continue to be the early-access advantage in fast turnaround fields: It is not that making findings accessible earlier merely gets them their citation “quota” earlier; providing OA earlier significantly increases that quota, probably by both accelerating and broadening the uptake of the findings in further research 
. And even after the competitive advantage is gone because all articles are OA, the download advantage
will continue to be enjoyed by all articles 
(thereby potentially influencing research even where it does not generate citations), while the quality advantage will see to it that for the best work, increased downloads are translated into uptake, usage and eventual increased citations. (Higher download counts earlier on have been found to be correlated with, hence predictive of, increased citation counts later; 
Summary and Conclusion
The assumption that increasing access to research will increase its usage and impact is the main rationale for the worldwide OA movement. Many prior studies have by now shown across all fields that journal articles that are made freely accessible to all potential users are cited significantly more than articles that are accessible only to subscribers. There is prior evidence for a self-selection bias toward the preferential self-archiving of higher quality articles in a few special fields (such as astronomy and some areas of physics) where most articles are made OA in unrefereed preprint form long before they are refereed and published, and where the published version is effectively accessible to all potential users as soon as it is published. Authors may indeed be more reluctant to make the preprints of papers about which they have doubts freely accessible online before they are refereed 
. But we have now shown that for most other fields (i) the OA Advantage remains just as high for mandatory self-archiving as for self-selected self-archiving and that (ii) this is not an artifact of systematic biases in other correlates of citation counts. Both the self-archiving and the mandates apply to refereed postprints, upon acceptance for publication, not to unrefereed preprints.
Hence the OA Advantage is real, independent and causal. It is indeed true that the size of the advantage is correlated with quality, just as citations themselves are correlated with quality (the top 20% of articles receiving about 80% of all citations); but we infer that the real cause of the higher OA advantage for the more citable articles is not a quality bias from author self-selection but the quality advantage of the more citable articles, an advantage that OA enhances by maximizing accessibility, and thereby also citability. On a playing field leveled by OA, users can selectively access, use and cite those articles that they judge to be of the highest relevance and quality, no longer constrained by their accessibility.
Overall, only about 15–20% of articles are being spontaneously self-archived today, self-selectively. To reach 100% OA globally, researchers' institutions and funders need to mandate self-archiving, as they are now increasingly beginning to do
. We hope that this demonstration that the OA Impact Advantage is real and causal will provide further incentive and impetus for the adoption of OA mandates worldwide in order to ensure that research can at last achieve its full impact potential, no longer constrained by today's needless limits on its accessibility to its intended users 
To measure that maximized research impact, we and others are already developing new OA metrics for monitoring, analyzing, evaluating, crediting and rewarding research productivity and progress 
. Hence there is no need to have any penalties or sanctions for non-compliance with OA self-archiving mandates. As the experience of Southampton ECS, Minho, QUT and CERN has already demonstrated, OA mandates, together with OA's own intrinsic rewards (enhanced research access, usage and impact), will be enough to reinforce the causal connection between providing access and reaping its impact, through the research community's existing system for evaluating and rewarding research productivity. In the online era, researchers' own “mandate” will no longer just be “publish-or-perish” but “self-archive to flourish.”