Self-selected or mandated, open access increases citation impact for higher quality research.

Background: Articles 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.