Commissioned Reports

Measuring Open Access Uptake: Data Sources Expectations and Misconceptions

Authors: {'first_name': 'Nicolas', 'last_name': 'Robinson-Garcia'},{'first_name': 'Thed N.', 'last_name': 'van Leeuwen'},{'first_name': 'Daniel', 'last_name': 'Torres-Salinas'}


In this paper we briefly introduce the concept of Open Access and review the many variants that have been presented in the literature. We then critically examine how OA variants are presented by data source and how they are operationalized in practice. The goal of the paper is to provide a set of guidelines on how to effectively interpret OA information. To this end, we compare OA figures reported in different data sources at the institutional and journal level and dig into the potential explanations behind the differences observed on the figures each source provides.

Policy highlights

Keywords: open accessunpaywallopen scienceresearch evaluationscientometrics 
 Accepted on 06 Oct 2020            Submitted on 22 Jul 2020


Open Access (OA) is now completely integrated into the research policies of funding agencies, governments, and institutions. According to the Budapest Open Access Initiative (Chan et al., 2002), we understand OA as:

“free availability on the public internet, permitting any users to read, download, copy, distribute, print, search, or link to the full texts of these articles, crawl them for indexing, pass them as data to software, or use them for any other lawful purpose, without financial, legal, or technical barriers other than those inseparable from gaining access to the internet itself.”

OA is a central player within the European Commission’s effort to make Open Science a reality. Still, the controversy surrounding its implementation has never stopped. The constant presence of predatory journals which led many to relate OA publishing with low research quality (Bohannon, 2013) is still there, and heated debates on how OA must be implemented continue with Plan S1 still pending over our heads (Haug, 2019). But some things have changed. Publishers are no longer fighting against its expansion but are pressing to be included in future OA publishing models. Also, since a couple of years, we can now actually provide metrics on OA uptake. These two changes introduce more complexity into OA discussions. They do so for two reasons. First, publishers are introducing different variants of OA which do not strictly fit into the definition of OA as exposed in the Budapest Open Access Initiative (BOAI) (Chan et al., 2002). Second, the fact that OA can now be quantified serves as evidence of its success or failure, including analyses of the different types of OA. But the way in which these metrics are computed also partially diverges from what it is theoretically considered as OA.

The purpose of this paper is to clarify and discuss these issues and show how information on OA data can be extracted and critically examined so that we can provide a fair and accurate overview of OA uptake in our analyses. For this we will discuss the following points. First, we will briefly review the different types of OA discussed in the literature, including those more controversial variants which do not necessarily conform with the requirements presented in the BOAI to be considered as OA.

Second, we will discuss the various efforts made in the last years to quantify and identify the extent to which scientific literature is available in OA, from the first studies which inferred the total number of OA scientific literature based on various methods of data sampling to the latest ones which use large datasets of publications in an attempt to make comprehensive estimates based on as much scientific literature as possible. Next, we will focus on Unpaywall (Piwowar et al., 2018), the most widespread tool currently available for identifying OA literature. This tool is not only used by many scholars, librarians and students as a means to access publicly available research papers, but it has been implemented in the major multidisciplinary scientific literature databases (e.g., Web of Science, Scopus, Dimensions) as well as in the Leiden Ranking (van Leeuwen et al., 2019). However, until recently, this tool provided only data on the location of OA versions of documents but did not define the type of OA. Hence the ways in which these OA types have been defined in these different sources (and even now by Unpaywall) differ slightly from one another. Furthermore, there are some technicalities in the way Unpaywall operates which can lead to misconceptions in many cases.

In the last section we will provide some examples using data from different sources on how OA indicators can be built ad hoc and analysed at the institutional level. For this we will compare the results reported by different data sources and discuss their differences.

Open Access: The canonical definition and its many variants

A scientific publication is considered as OA when it complies with the following requirements: 1) free availability; 2) allows users to read, download, copy, distribute, print, search or link to the full text; 3) allows crawling the full text for indexing; and 4) imposes no legal, financial or technical barriers (Chan et al., 2002). To ensure that a scientific publication becomes OA two strategies were originally proposed: self-archiving in repositories or publishing in OA journals (Harnad et al., 2008). These two strategies are known as green OA and gold OA and refer to the venue by which OA is enabled. Over time many other variants have been defined, not only based on the venue but also on the level of openness. For instance, (Suber, 2012) proposes distinguishing between gratis OA (access free of costs while retaining some restrictions) and libre OA (access free of costs and restrictions)-always from a reader perspective.

However, most of the discussions on OA variants are related with the business model that enables the access. Along this line, hybrid OA refers to papers published as OA in subscription-based toll journals but for which the authors have paid an extra fee to make their paper freely available. Diamond OA is proposed for non-profit OA journals which do not follow an author pays model (Fuchs & Sandoval, 2013). Black OA refers to illegal services which offer free access to scientific publications such as LibGen or Sci-Hub (Björk, 2017). But the complexity of the variants is even greater, with some subsidized gold OA journals being partly diamond, as the publisher itself partly funds the APC costs.

Laakso et al. (2011) propose alternative criteria to establish OA variants in which the business model and the time of access are intertwined. In their approach all variants refer to OA publishing and do not include self-archiving. They distinguish between direct OA, which refers to publishing in OA journals; delayed OA, which refers to journals liberating their contents after an embargo period (Laakso & Björk, 2013); and, the aforementioned hybrid OA.

An additional type is the so-called bronze OA. Unlike the other types, this variant does not respond to a theoretical definition but is the result of empirically attempting to categorize OA publications by type. Bronze OA was first defined by Piwowar et al. (2018) as “free to read on the publisher page, but without a clearly identifiable license” (p. 5). In their paper, they offer a general overview of OA uptake. They distinguish between green, gold, hybrid, and bronze OA. The bronze OA definition reflects the difficulty of empirically assigning OA types to papers when a very specific criterion is followed, as is basically a basket concept in which OA papers are placed which do not fall under the other categories. Bronze OA could relate to delayed OA as defined before, but also to specific publications that journals decide to make openly accessible for various reasons, such as a worldwide health urgency (e.g., the COVID-19 pandemic), for promotional campaigns (e.g., Springer Nature makes a selection of articles published by Nobel Prize laureates freely accessible2), or emerging from a sense of responsibility (e.g., PNAS and The New England Journal of Medicine make their contents freely accessible after six months of publication).

But this is not the only case where it is empirically difficult to assign OA types to papers. Another problematic OA type to empirically assess is gold or APC OA. This refers to OA journals for which the author must pay a fee (generally referred to as Author Processing Charges or APCs). The best data source from which to extract APC information on OA journals is the Directory of Open Access Journals or DOAJ.3 This journal directory however, presents some shortcoming discussed in the literature. First, it may not result in a complete picture of Gold OA publishing(Björk, 2019; Pölönen et al., 2019). Second, the year a journal provided online Open Access content may differ from the real one(Bautista-Puig et al., 2020). As we noted in a previous study:

After some inspection, we found some inconsistencies in the way APC is defined according to DOAJ. That is, not in all cases, APC refers to an author pays model, but in some cases, journals offer an optional subscription fee for those interested on accessing to printed versions of the journal. This is the case for many journals stored in the SciELO platform which are free of costs for both readers and authors, but which offer the option to pay a subscription fee for printed versions of the journal. (Robinson-Garcia et al., 2020, p. 5)

Some attempts have been made to improve the coverage of OA journals (Rimmert et al., 2017). But the extent to which it does so has not been explored. Additional examples can probably be found when attempting to empirically categorize OA publications. For instance, the appearance of multiple versions of the same document can mislead readers who may not be certain if they are reading the final version of the manuscript or previous versions (van Leeuwen et al., 2018). These examples illustrate the difficulties one is confronted with when operationalizing some of the definitions of OA and its variants.

How much scientific literature is openly accessible and how do we measure it?

A long-standing debate for many years relates to the amount of scientific literature which is available in OA. In a first instance, studies estimating OA uptake sample a set of scientific papers and infer from it the share of papers OA for the whole population (Archambault et al., 2014; Björk et al., 2010; Gargouri et al., 2012). As the methodology and the way OA was operationalized varied, estimates also differ. OA literature in its broadest sense (that is, understood as free to read access) was estimated to represent half of the scientific literature in 2013 (Kaiser, 2013; Van Noorden, 2013), while more restricted estimates ranged between 20% and 25%. For a more thorough discussion on how methodological decisions affect results we refer to the study by van Leeuwen et al. (2018).

A more comprehensive approach was made possible more recently when large-scale analyses became easier to conduct as computational advances on data retrieval and processing evolved. Two studies are worth mentioning. First, a large-scale analysis in which publications indexed in Web of Science were matched against Google Scholar to identify free to read versions of the documents (Martín-Martín et al., 2018). The authors used a similar approach to that adopted by Archambault et al. (2014), in the sense that they did not restrict themselves to strictly identifying OA literature, but rather free to read literature. They searched for evidence of free access in Google Scholar for papers published either in 2009 or in 2014 (over 2.5 Million documents). Overall, they found that slightly more than half of the publications were free to read, always considering that the embargo period for delayed OA had probably expired by the time of the analysis. But the costs in terms of time and manpower needed to pursue this type of analysis have prevented it from being updated or implemented as a viable methodology for identifying OA.

The second study presents a large-scale analysis of OA uptake at universities (Robinson-Garcia et al., 2020). Here, the authors used the Unpaywall API and crossed it with Web of Science and the Leiden Ranking to obtain OA indicators for 963 universities worldwide during the time period 2014–2017. Over 4.5 million publications were analysed of which 40% were OA. In this case the share is lower than in the first large scale study mentioned above, probably due to a more restricted definition of OA (as defined by Unpaywall). The results of this study were implemented in the 2019th and 2020th editions of the Leiden Ranking (Centre for Science and Technology Studies (CWTS), n.d.).

OA metadata from Unpaywall

The real game changer on the identification of OA literature was Unpaywall (Piwowar et al., 2018). A tool developed by the non-profit Our Research,4 founded by Jason Priem and Heather Piwowar. Unpaywall can be queried through different means:

  1. An API, which can be queried by any user with some basic programming skills. In the Appendix we include a how-to guide to query it, using the R programming language.
  2. A personalized service for research purposes by which a user can introduce the DOIs of the set of papers she/he is interested, and later receive from the tool an email with the results.
  3. Using an add-on5 to the web browser in which the user can see in a side tab whenever they land on an article from the publishers’s website whether or not the article is available in Open Access.

The key advantage of Unpaywall is twofold. First it is viable in terms of cost. Second, its capacity to accurately identify OA has been unchallenged so far. Unpaywall relies heavily on DOI identifiers gathering its data on scientific literature from CrossRef,6 the official Digital Object Identifier (DOI) Registration Agency of the International DOI Foundation. This already imposes a major limitation on any OA analysis and may undermine OA presence in fields or for document types where DOIs are not as frequently used, e.g., in humanities fields, and for monographs or reports.

Unpaywall has now been implemented in all the major bibliometric databases, including Scopus, Web of Science and Dimensions. However, each data source defines OA variants differently. Table 1 shows the different OA types defined in each source including Unpaywall, reflecting a high degree of inconsistency between them. Furthermore, and probably more worrisome, they define OA types as exclusive categories. This affects especially green OA, as it is the only category which is not journal-dependent and hence can be combined with any of the other types. In this sense, all data sources label a publication as green OA only if none of the other types of OA applies to this publication. If another type of OA does apply as well, it is labelled as that other type, not as green OA, thus undermining the uptake of green OA Intentionally or not, this practice hides institutions’ and researchers’ efforts in actively ensuring OA through the development and support of repositories and provides a biased view of OA to the reader of such figures, suggesting that this type of OA is not as common when it is actually the most common OA type.

Table 1

Types of Open Access provided by four different data sources.

OA type Unpaywall Web of Science Scopus* Dimensions

All OA X X X
Closed X
Open Access X
Other X
Bronze X X
Hybrid X
Gold X X
    DOAJ Gold X
    Other Gold X
Green X
    Green, Published X X
    Green, Accepted & Submitted X
    Green, Accepted X

* Crosses (X) indicates differences in the extent to which a particular OA type is covered.

Note: Scopus defines in its web interface Open Access as “Articles published in Gold OA, including full journals, Hybrid, Open Archive and Promotional Access” and Other as “any other type of access including Subscription of Green OA (not yet supported in Scopus)”.

In Figure 1 we provide an example on differences of green OA depending on how categories are defined (as exclusive or inclusive). The COVID-19 publications dataset compiled by Dimensions (Dimensions COVID-19 Publications, Datasets and Clinical Trials, 2020) was downloaded at three different points in time during May 2020. Only journal articles and preprints were used in this example. After downloading them, the Unpaywall API was queried to establish the OA type of each of the documents downloaded. Four OA types are showcased: gold, bronze, hybrid, and green OA. In the case of the latter, this is shown twice, the first green OA bar indicates the number of documents which are green OA according to Unpaywall’s categorization and we refer to it as green (Unpaywall). The second one reports all instances of green OA, regardless of double assignments, we refer to it as green (RCvL). As observed, an analysis based on the Unpaywall categories would suggest that most of the OA COVID-19 publications are made available through the publishers as bronze OA. However, the largest bunk of OA COVID-19 literature is available via green OA.

Figure 1 

Differences on OA uptake for COVID-19 literature at three points in time. Data on COVID-19 publications from Dimensions (Dimensions COVID-19 Publications, Datasets and Clinical Trials, 2020). Open Access data retrieved from the Unpaywall API. Gold, Bronze, Hybrid and Green (Unpaywall) retrieved from the ‘status’ field from Unpaywall API. Green (RCvL) OA computed as defined by (Robinson-Garcia et al., 2020).

The volatility problem

We must note the volatility of these data sources and the limitations of being overly accurate on OA numbers. Again, using the COVID-19 publications data, we looked at changes of OA status and growth at the three points in time in which the data was extracted. Of course, the case of COVID-19 literature is a special case study, where there has been an overwhelming increase of publications in a very short time (Brainard, 2020; Torres-Salinas, 2020). Still, we believe it reflects very well issues of stability with OA data. Figure 2 shows fluctuations of papers present as well as the addition of new publications at the three points in time. In principle, except for bronze OA, OA status should not change over time. A paper published in a gold OA journal should remain as gold OA. The same goes for hybrid OA. In the case of green OA, and when using an exclusive criterion, it can only shift to other OA types or remain as green. However, we observe that this is not the case, and papers reported at one point as OA, are reported as “closed” in later stages. Although the number of papers which change their status are marginal (below 50), still these inconsistencies illustrate the volatile nature of scientific literature and the magnitude of the task that Unpaywall fulfils. These limitations do not in any way discard Unpaywall as an OA discovery tool or as a tool to monitor OA uptake, but must be considered when reporting Unpaywall-based data. The cause for these changes remains to be explored.

Figure 2 

Changes of OA status for COVID-19 literature at three points in time. Data on COVID-19 publications from Dimensions (Dimensions COVID-19 Publications, Datasets and Clinical Trials, 2020). Open Access data retrieved from the Unpaywall API. OA types as defined by Unpaywall API. Curves indicate displacements of particular journals from one OA category to another.

Using secondary sources to monitor OA uptake

Multidisciplinary Citation Indexes still represent the easiest and most common way to report and monitor OA literature. Nevertheless, issues should be considered not only relating to the processing of OA metadata but also to the coverage of the data sources and other decisions that may affect the results. Figure 3 looks at the number of OA publications of three universities in the 2015–2018 period according to three data sources: Web of Science, Scopus, and the Leiden Ranking. While it was expected to see differences on raw numbers by source, the magnitude of such differences differs by source. While Web of Science covers the largest number of OA publications for all three institutions, the Leiden Ranking obtains the second position in the results for the two Dutch universities, but the third with respect to the University of Granada. Differences between the Leiden Ranking and Web of Science (which in principle depart from the same corpus of literature) relate to the document type (in the Leiden ranking only citable documents are included) as well as differences in the institution name disambiguation process. In the case of the University of Granada, the lower share of OA according to the Leiden Ranking may also originate from the fact that this source excludes the Arts & Humanities Citation Index and publications written in non-English languages which may affect this university stronger than the other two.

Figure 3 

Number of OA publications for three universities in the 2015–2018 period according to data from the Leiden Ranking, Scopus and Web of Science. Institutions were searched using the organization-enhanced option both in Scopus and Web of Science. No filters by document type were applied.

These differences are key to the interpretation made in any analysis and illustrate the need to combine different sources to give an accurate picture of institutional uptake. In the case of the Leiden Ranking, the fact that the complete dataset is made freely available (Van Eck, 2020) makes the data processing and retrieval easier, and offers an interesting benchmark for comparison with other institutions. On the other hand, its use may lead to unexpected divergences, as differences in publication counts among data sources are not merely due to OA capturing, but also due to source journal coverage, document type selection and affiliation handling.

In Table 2 we compare datasets which in principle should show similar figures. That is, we present a comparison of publications and OA uptake for a list of journals in Scopus and Web of Science. In this case, we only include articles and reviews. This already yields some problems, as there are several discrepancies between databases. As observed, Web of Science always reports a higher share of OA publications with notable differences in the cases of JAMA, Annals of Internal Medicine and New England Journal of Medicine. In these three cases, Web of Science reports an increase of 39%, 33% and 35% of OA publications respectively. This is due to the inclusion of Unpaywall in which a broader array of OA types is included, especially Green OA, which is not included in Scopus. When comparing only gold OA (which includes hybrid periodicals), journals report similar figures regardless of the database.

Table 2

Number and percentage of OA publications for 20 medical journals in Scopus and Web of Science. Only articles and reviews are included. Time period 2015–2018.

Scopus PUBS % Total OA WoS PUBS % Total OA Type of Open Access


BMJ OPEN 8543 98% 8493 100% 98% 100% 0%
NEW ENGLAND JOURNAL OF MEDICINE 2324 31% 1318 66% 52% 0% 47%
LANCET 1352 19% 1212 43% 41% 15% 4%
EBIOMEDICINE 1129 100% 1133 100% 100% 100% 0%
ANNALS OF INTERNAL MEDICINE 1026 0% 567 33% 33% 0% 0%
BMC MEDICINE 816 100% 817 100% 100% 100% 0%
PLOS MEDICINE 800 100% 672 100% 100% 100% 0%
INTERNAL MEDICINE JOURNAL 790 9% 761 12% 4% 1% 8%
FRONTIERS IN MEDICINE 663 94% 716 100% 99% 100% 0%
ARCHIVES OF MEDICAL SCIENCE 649 96% 652 100% 96% 100% 0%
SWISS MEDICAL WEEKLY 609 92% 488 100% 43% 100% 0%
MAYO CLINIC PROCEEDINGS 575 44% 575 61% 28% 6% 37%
ANNALS OF MEDICINE 299 7% 296 17% 15% 3% 3%
OXFORD MEDICAL CASE REPORTS 261 99% 343 100% 99% 100% 0%
EUROPEAN JOURNAL OF GENERAL PRACTICE 151 83% 152 88% 62% 68% 12%

Concluding remarks

In this paper we briefly introduce the concept of OA and confront theoretical definitions and conceptions with its operationalization to provide the reader with a critical mindset when analysing and interpreting figures reporting OA uptake. Furthermore, we inform on how to retrieve and use OA metadata for any given set of publications either through the Unpaywall API or via other available data sources. We refer specifically to Web of Science, Scopus, Dimensions, and the Leiden Ranking. We compare them and critically comment on the information each source reports and on potential reasons for disparities. The analyses shown are illustrative and not in-depth analyses, and should be interpreted as such.

OA is currently a heated and debated topic, both in the field of scientometrics and in university management. The different stakeholders involved feel passionately about it, claiming in favour or against the many initiatives that are taking place. The launch of Unpaywall has provided for the first-time numbers which go beyond rough estimates. But still the way in which they are calculated and interpreted can greatly mislead decisions and opinions. With this paper we hope to bring some insight which can nurture informed and reasoned discussions on the future direction of OA. While we do not respond to all questions related to the operationalization of OA and its interpretation, we hope we provide a brief introduction so that more insightful analyses can take place in the future.

Issues such as licensing of OA publications and their adherence to national or supranational policies, or the inclusion of additional publication types such as monographs or grey literature are not discussed in this paper but still pose important challenges in terms of effectively measuring OA uptake and reporting compliance with policies. The reliance of document identifiers (e.g., DOI, HANDLE) for any OA tracking tool, including Unpaywall will impose an important barrier when trying to go beyond the domain of journal articles.


1Plan S is an initiative led by Science Europe (n.d.) with the goal of transitioning to a full OA science by requiring all funded research to be OA. 

Appendix: How to extract and process OA metadata from Unpaywall API

Here we provide a brief guide on how to retrieve OA metadata for any given set of publications by querying the Unpaywall API.

The Unpaywall API allows us to retrieve metadata, not only on the OA status and OA type of publications, but also a thorough description of OA evidence. This includes among others the URL location of the OA version of the document, the type of license of the document, and whether an OA version has been identified in a repository or not. It also includes some bibliographic information such as journal, document type, publisher, authors, or date of publication. The richness of the data provides a wide array of opportunities for analysis, some of them explored in the study by Robinson-Garcia et al. (2020), such as alternative definitions of OA variants, identification of green OA by repository (this can be explored within allocations), licensing of OA publications, etc.

For the extraction of data we will use the roadoi package (Jahn, 2020). This package queries the Unpaywall API by providing DOIs of the publications for which we want to know their OA status. While it includes a function by which DOIs can be queried one at a time, it does allow querying up to 100,000 publications at a time. For this, we suggest computing the function showcased in Figure S1.

Figure S1 

Unpaywall function to retrieve OA metadata from the Unpaywall API. In red and bold are highlighted the parts of the code which must be modified by the user. In this case, the user must include their email account. The roadoi should have been previously installed.

Next, we will retrieve our list of DOIs and copy paste them into a list using the following command:

list_of_dois <- c(“doi1”, “doi2”,…)

Hence, we will be able to compute the unpaywall() function and retrieve for each publication all the OA metadata available as retrieved by Unpaywall. The retrieved metadata is obtained in JSON format, meaning that some fields will contain additional variables. A full definition of the variables included is available at


This study shows some preliminary results from the project ‘Scientific communication in times of Corona virus’ funded by the TU Delft COVID-19 Response Fund. Nicolas Robinson-Garcia has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 707404. Daniel Torres-Salinas has received funding from the University of Granada’s “Plan Propio de Investigación y Transferencia” under the “Reincorporación de Jóvenes Doctores” Programme.

Competing Interests

The authors have no competing interests to declare.


  1. Archambault, É., Amyot, D., Deschamps, P., Nicol, A., Provencher, F., Rebout, L., & Roberge, G. (2014). Proportion of open access papers published in peer-reviewed journals at the European and world levels—1996–2013. 

  2. Bautista-Puig, N., Lopez-Illescas, C., de Moya-Anegon, F., Guerrero-Bote, V., & Moed, H. F. (2020). Do journals flipping to gold open access show an OA citation or publication advantage? Scientometrics, 124(3), 2551–2575. DOI: 

  3. Björk, B.-C. (2017). Gold, green, and black open access. Learned Publishing, 30(2), 173–175. DOI: 

  4. Björk, B.-C. (2019). Open access journal publishing in the Nordic countries. Learned Publishing, 32(3), 227–236. DOI: 

  5. Björk, B.-C., Welling, P., Laakso, M., Majlender, P., Hedlund, T., & Guðnason, G. (2010). Open Access to the Scientific Journal Literature: Situation 2009. PLOS ONE, 5(6), e11273. DOI: 

  6. Bohannon, J. (2013). Who’s Afraid of Peer Review? Science, 342(6154), 60–65. DOI: 

  7. Brainard, J. (2020). Scientists are drowning in COVID-19 papers. Can new tools keep them afloat? Science. DOI: 

  8. Centre for Science and Technology Studies (CWTS). (n.d.). CWTS Leiden Ranking. Centre for Science and Technology Studies (CWTS). Retrieved 11 September 2020, from 

  9. Chan, L., Cuplinskas, D., Eisen, M. B., Friend, F., Genova, Y., Guédon, J.-C., Hagemann, M., Harnad, S., Johnson, R., Kupryte, R., La Manna, M., Rev, I., Segbert, M., De Souza, S., Suber, P., & Velterop, J. (2002, February 14). Budapest Open Access Initiative. 

  10. Dimensions COVID-19 publications, datasets and clinical trials. (2020). [Data set]. Dimensions. 

  11. Fuchs, C., & Sandoval, M. (2013). The diamond model of open access publishing: Why policy makers, scholars, universities, libraries, labour unions and the publishing world need to take non-commercial, non-profit open access serious. TripleC: Communication, Capitalism & Critique, 11(2), 428–443. DOI: 

  12. Gargouri, Y., Lariviere, V., Gingras, Y., Carr, L., & Harnad, S. (2012, September). Green and Gold Open Access percentages and growth, by discipline. 17th International Conference on Science and Technology Indicators (STI). 

  13. Harnad, S., Brody, T., Vallières, F., Carr, L., Hitchcock, S., Gingras, Y., Oppenheim, C., Hajjem, C., & Hilf, E. R. (2008). The Access/Impact Problem and the Green and Gold Roads to Open Access: An Update. Serials Review, 34(1), 36–40. DOI: 

  14. Haug, C. J. (2019). No Free Lunch—What Price Plan S for Scientific Publishing? New England Journal of Medicine, 380(12), 1181–1185. DOI: 

  15. Jahn, N. (2020). rOpenSci: The roadoi package [R]. 

  16. Kaiser, J. (2013). Half of All Papers Now Free in Some Form, Study Claims. Science, 341(6148), 830–830. DOI: 

  17. Laakso, M., & Björk, B.-C. (2013). Delayed open access: An overlooked high-impact category of openly available scientific literature. Journal of the American Society for Information Science and Technology, 64(7), 1323–1329. DOI: 

  18. Laakso, M., Welling, P., Bukvova, H., Nyman, L., Björk, B.-C., & Hedlund, T. (2011). The Development of Open Access Journal Publishing from 1993 to 2009. PLOS ONE, 6(6), e20961. DOI: 

  19. Martín-Martín, A., Costas, R., van Leeuwen, T., & Delgado López-Cózar, E. (2018). Evidence of open access of scientific publications in Google Scholar: A large-scale analysis. Journal of Informetrics, 12(3), 819–841. DOI: 

  20. Piwowar, H., Priem, J., Larivière, V., Alperin, J. P., Matthias, L., Norlander, B., Farley, A., West, J., & Haustein, S. (2018). The state of OA: A large-scale analysis of the prevalence and impact of Open Access articles. PeerJ, 6, e4375. DOI: 

  21. Pölönen, J., Guns, R., Kulczycki, E., Laakso, M., & Sivertsen, G. (2019). Open access challenge at national level: Comprehensive analysis of publication channels used by Finnish researchers in 2016–2017. 17th International Conference on Scientometrics and Informetrics, ISSI 2019-Proceedings Volume 2, 2019. 

  22. Rimmert, C., Bruns, A., Lenke, C., & Taubert, N. C. (2017). ISSN-Matching of Gold OA Journals (ISSN-GOLD-OA) 2.0. 

  23. Robinson-Garcia, N., Costas, R., & van Leeuwen, T. N. (2020). Open Access uptake by universities worldwide. PeerJ, 8, e9410. DOI: 

  24. Science Europe. (n.d.). ‘Plan S’ and ‘cOAlition S’ – Accelerating the transition to full and immediate Open Access to scientific publications. Retrieved 11 September 2020, from 

  25. Suber, P. (2012). Open Access. MIT Press. DOI: 

  26. Torres-Salinas, D. (2020). Ritmo de crecimiento diario de la producción científica sobre Covid-19. Análisis en bases de datos y repositorios en acceso abierto. El profesional de la información (EPI), 29(2). DOI: 

  27. Van Eck, N. J. (2020). CWTS Leiden Ranking 2020 [Data set]. Zenodo. DOI: 

  28. Van Leeuwen, T. N., Costas, R., & Robinson-Garcia, N. (2019, May 15). Indicators of open access publishing in the CWTS Leiden Ranking 2019. CWTS Blog. 

  29. Van Leeuwen, T. N., Tatum, C., & Wouters, P. F. (2018). Exploring possibilities to use bibliometric data to monitor gold open access publishing at the national level. Journal of the Association for Information Science and Technology, 69(9), 1161–1173. DOI: 

  30. Van Noorden, R. (2013). Half of 2011 papers now free to read. Nature News, 500(7463), 386. DOI: