May 18, 2022

Research Article: “The State of OA: a Large-Scale Analysis of the Prevalence and Impact of Open Access Articles” (Peer-Reviewed Version)

UPDATE March 12 “‘Bronze’ Open Access Supersedes Green and Gold” (by John Brock ; via Nature Index)

A Nature Index report about the research article linked below. The article includes two interactive charts and comments from:

  • Heather Piwowar, Impactstory (and Co-Author of Article)
  • Thed van Leeuwen, Centre for Science and Technology Studies, Leiden University,



Note: The peer-reviewed version of “The State of OA: a Large-Scale Analysis…” is now available online and liked below.

A preprint version of this article was released last August.


The State of OA: A Large-Scale Analysis of the Prevalence and Impact Of Open Access Articles


Heather Piwowar​​

Jason Priem​​

Vincent Larivière
Université de Montréal
CIRST, Université du Québec à Montréa

Juan Pablo Alperine
Simon Fraser University
Public Knowledge Project

Lisa Matthias
Independent Scholar

Bree Norlander
University of Washington

Ashley Farley
University of Washington

Jevin West
University of Washington

Stefanie Haustein
University of Ottawa
CIRST, Université du Québec à Montréal


PeerJ 6:e4375
doi: 10.7717/peerj.4375


Despite growing interest in Open Access (OA) to scholarly literature, there is an unmet need for large-scale, up-to-date, and reproducible studies assessing the prevalence and characteristics of OA.

We address this need using oaDOI, an open online service that determines OA status for 67 million articles. We use three samples, each of 100,000 articles, to investigate OA in three populations: (1) all journal articles assigned a Crossref DOI, (2) recent journal articles indexed in Web of Science, and (3) articles viewed by users of Unpaywall, an open-source browser extension that lets users find OA articles using oaDOI. We estimate that at least 28% of the scholarly literature is OA (19M in total) and that this proportion is growing, driven particularly by growth in Gold and Hybrid. The most recent year analyzed (2015) also has the highest percentage of OA (45%).

Because of this growth, and the fact that readers disproportionately access newer articles, we find that Unpaywall users encounter OA quite frequently: 47% of articles they view are OA. Notably, the most common mechanism for OA is not Gold, Green, or Hybrid OA, but rather an under-discussed category we dub Bronze: articles made free-to-read on the publisher website, without an explicit Open license. We also examine the citation impact of OA articles, corroborating the so-called open-access citation advantage: accounting for age and discipline, OA articles receive 18% more citations than average, an effect driven primarily by Green and Hybrid OA. We encourage further research using the free oaDOI service, as a way to inform OA policy and practice.

Direct to Full Text Article

Note: Heather Piwowar and Jason Priem (two of the authors) are the founders of ImpactStory and the developers of the Unpaywall browser add-on and oaDOI API service, both of these services can make easy access open acces articles, preprints, etc. much easier for most users.

Priem was also a co-author of the altmetrics manifesto in 2010.

Priem was also a co-author of the altmetrics manifesto in 2010.

About Gary Price

Gary Price ( is a librarian, writer, consultant, and frequent conference speaker based in the Washington D.C. metro area. Before launching INFOdocket, Price and Shirl Kennedy were the founders and senior editors at ResourceShelf and DocuTicker for 10 years. From 2006-2009 he was Director of Online Information Services at, and is currently a contributing editor at Search Engine Land.