Table 3. Studies included in the meta-synthesis: Methodological approaches.

Study	Data sources	No. of analysed publications	Publication years	Definition of open access
Larivière and Sugimoto (2018)	Papers published between 2009 and 2017 that are indexed in WoS and have a DOI, combined with Unpaywall	12,683,296	2009 – 2017	Articles freely available to read, with two non-exclusive subcategories: Gold (available on a journal website) and Green (available in a repository). Bronze, Hybrid, Delayed and ASN/Other free availability not included.
Piwowar et al. (2018)	Random sample of recent journal articles indexed in WoS and with DOIs, combined with oaDOI database	100,000 articles	2009 – 2015	Publications free to read online, with four exclusive sub-categories: Gold (published in an OA journal that is indexed in the DOAJ); Green (toll-access on the publisher page, but with a free copy in an OA repository); Hybrid (free under an open license in a toll- access journal); Bronze (free to read on the publisher page, but without open license). ASN/Other free availability not included.
Bosman and Kramer (2018)	Full WoS database, combined with oaDOI database	12.3 million articles and reviews	2010 – 2017	Publications free to read online, with four exclusive sub-categories: Gold (published in an OA journal that is indexed in the DOAJ); Green (toll-access on the publisher page, but with a free copy in any OA repository); Hybrid (free under an open license in a toll-access journal) and Bronze (free to read on the publisher page, but without identifiable license). ASN/ Other free availability not included.
Science-Metrix (2018)	All articles in WoS and Scopus, combined with 1science database of OA articles	13.2 million articles	2006 – 2015	Articles available on the Internet in full-text form, that are freely readable and downloadable, with two non- exclusive sub-categories: Gold (made available for free by the publishers themselves, containing pure Gold, Bronze and Hybrid, or on the side of an aggregator) and Green (available in any repositories by parties other than publishers, includes Green only and ASN/Other free availability).
Martín-Martín et al. (2018)	All documents with a DOI from WoS, Social Sciences Citation Index and Arts & Humanities Citations Index, combined with GS	2.6 million documents	2009 and 2014	Freely online available publications, with four exclusive sub-categories: Gold (available in pure Gold journals listed in the DOAJ); Hybrid (available in journals not listed in the DOAJ, but with an OA license effective at the time of publication); Bronze (available in journals not listed in the DOAJ and without OA license); Green (available in institutional or subject repositories listed in ROAR and OpenDOAR); Delayed (available in journals with an embargo period) and ASN/Other free availability (available on websites, ASN, harvesters).
Jamali and Nabavi (2015)	First ten hits from queries of minor Scopus subject categories in GS	7,244 articles	1996 – 2013	Any free full-text version of articles accessible through GS, with two exclusive sub-categories (Gold and Green, not explicitly defined). No information on Bronze, Delayed, or Other free availability.
Khabsa and Giles (2014)	GS	Capture- recapture approach	No limit	Any free full-text version of articles accessible through GS, with two exclusive sub-categories (Gold and Green, not explicitly defined). No information on Bronze, Delayed, or ASN/Other free availability.
Archambault et al. (2014)	Scopus, combined with searches of DOAJ, ROAR, OpenDOAR, PubMedCentral, and other sources of freely downloadable papers	513,753 articles	1996 – 2013	Articles freely available to all, with three exclusive sub-categories: Gold (available in journals listed in the DOAJ, and on the PubMed Central list of journals); Green (available on institutional repositories as listed in OpenDOAR and/or in ROAR) and ASN/Other free availability (Delayed, Hybrid, available on authors’ web pages and elsewhere, in ASN and on aggregator sites).
Gargouri et al. (2012)	Random samples of articles indexed in Thomson- Reuters-ISI, combined with robot crawling web for OA full- texts	107,052 articles	1998 – 2006 and 2005 – 2010	Articles freely accessible online, with two exclusive sub-categories: Gold (articles freely accessible online in a journal) and Green (self-archived online and free for all copies of published work in any appropriate journal). No information on Bronze, Delayed, or ASN/Other free availability.
Björk et al. (2010)	Random sample of articles from Scopus, combined with Google searches for OA full-texts	1,837 articles	2008	Access to articles without any restrictions posed by subscriptions, with two exclusive sub-categories: Gold (articles published directly in OA journals) and Green (articles posted openly in any repositories or other web sites). ASN/Other free availability included as "other web sites" in Green. No information on Bronze or Delayed.
Hajjem (2006)	CDROM version of ISI’s Science and Social Science Citation Indices, combined with robot crawling of the web for OA full-texts	1,307,038 articles	1992 – 2003	Any full text accessible on the web, no sub-categories defined.