Green OA consists of researchers continuing to publish in traditional subscription journals, and then self-archiving their final peer-reviewed papers on the Web, either in an institutional repository or in a central or subject-based repository like arXiv or PubMed Central. In this way they can ensure that any other researcher in the world is able to access their papers, regardless of whether the other researcher's institution has a subscription to the journals in which the papers are published.
Gold OA, by contrast, consists of researchers publishing in specialist OA journals (e.g. the journals of OA publishers like BioMed Central or Public Library of Science) rather than in a subscription journal. Instead of limiting access by imposing a subscription, OA publishers levy on authors (or more usually their funders or institutions) an article processing charge (APC). This is intended to cover the costs of organising peer review, and any costs incurred in making papers available online.
There are two variations on Gold OA: Hybrid OA, where a subscription journal agrees to make single papers freely available on the Web on payment of an APC (while the rest of the papers remain available to subscribers only); and institutional "membership" OA, where a research institution forward-buys in bulk the right for all of its researchers to publish with a specific OA publisher, thereby avoiding the need to pay an APC every time an article is published.
Whether Green or Gold, OA implies providing immediate, permanent, toll-free online access to the full-texts of peer-reviewed research journal articles.
The logic of OA is that the global network allows the scholarly community to dispense with the traditional paywalls characteristic of the print publishing world, thereby maximising the number of researchers able to view published papers. The assumption is that this will enable research to develop more quickly, and more effectively.
But as the OA movement has developed an interesting question has arisen: should Green and Gold OA be viewed as concurrent or consecutive activities?
This is not an issue of intellectual curiosity alone: it has important strategic implications for the OA movement. It requires, for instance, that the movement decides whether to treat Green and Gold OA as complementary or competitive activities; and if they are competitive, then where the OA movement should focus its main efforts ...
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Peter Suber comments on this article here.
Ivy Anderson, Director, Collections, California Digital Library comments here.
Former Commercial Director of publisher Wiley-Blackwell Steven Hall comments here
Stevan Harnad responds to Ivy Anderson's comments here.