ARTÍCULO
OpenCitations, an infrastructure organization
for open scholarship
Silvio Peroni1,2
and David Shotton2,3
1Digital Humanities Advanced Research Centre, Department of Classical Philology and Italian Studies,
University of Bologna, Bologna, Italia
2Research Centre for Open Scholarly Metadata, Department of Classical Philology and Italian Studies,
University of Bologna, Bologna, Italia
3Oxford e-Research Centre, Universidad de Oxford, Oxford, Reino Unido
Palabras clave: bibliographic metadata, infrastructure service, OpenCitations, OpenCitations Corpus,
OpenCitations Data Model, OpenCitations Index, OpenCitations Meta, open citations, Open
Citation Identifier, open scholarship, scholarly infrastructure organization
ABSTRACTO
OpenCitations is an infrastructure organization for open scholarship dedicated to the publication
of open citation data as Linked Open Data using Semantic Web technologies, thereby
providing a disruptive alternative to traditional proprietary citation indexes. Open citation data
are valuable for bibliometric analysis, increasing the reproducibility of large-scale analyses
by enabling publication of the source data. Following brief introductions to the development
and benefits of open scholarship and to Semantic Web technologies, this paper describes
OpenCitations and its data sets, herramientas, services, and activities. These include the OpenCitations
Data Model; the SPAR (Semantic Publishing and Referencing) Ontologies; OpenCitations’
open software of generic applicability for searching, browsing, and providing REST APIs over
resource description framework (RDF) triplestores; Open Citation Identifiers (OCIs) y el
OpenCitations OCI Resolution Service; the OpenCitations Corpus (OCC), a database of open
downloadable bibliographic and citation data made available in RDF under a Creative
Commons public domain dedication; and the OpenCitations Indexes of open citation data, de
which the first and largest is COCI, the OpenCitations Index of Crossref Open DOI-to-DOI
Citations, which currently contains over 624 million bibliographic citations and is receiving
considerable usage by the scholarly community.
INTRODUCCIÓN
1.
Bibliographic citations—the conceptual directional links from a citing bibliographic entity to a
cited bibliographic entity created when the author of a published work acknowledges other
works in its bibliographic references (peroni & Shotton, 2018a)—are one of the most funda-
mental types of bibliographic metadata and central to the world of scholarship. They knit to-
gether independent works of scholarship into a global endeavor and are important for
assigning credit to other researchers. The open availability of citation data is a crucial require-
ment for the bibliometrics and scientometrics domain because it “is essential to promote re-
producibility and appraisal of research, reduce misconduct, and ensure equitable access to
and participation in science” (Sugimoto et al., 2017).
Actualmente, the two most authoritative sources of citation data are Clarivate Analytics’ Web
de Ciencia ( WoS), which grew from the Science Citation Index created by Eugene Garfield in
un acceso abierto
diario
Citación: peroni, S., & Shotton, D.
(2020). OpenCitations, an infrastructure
organization for open scholarship.
Estudios de ciencias cuantitativas, 1(1),
428–444. https://doi.org/10.1162/
qss_a_00023
DOI:
https://doi.org/10.1162/qss_a_00023
Recibió: 27 Junio 2019
Aceptado: 09 December 2019
Autor correspondiente:
Silvio Peroni
silvio.peroni@opencitations.net
Handling Editors:
Ludo Waltman and Vincent Larivière
Derechos de autor: © 2020 Silvio Peroni and
David Shotton. Published under a
Creative Commons Attribution 4.0
Internacional (CC POR 4.0) licencia.
La prensa del MIT
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OpenCitations
1964, and Elsevier’s Scopus, launched in 2004. Neither are open; most research universities
have to pay tens of thousands of dollars annually to access one or both of them, whereas in-
stitutions and independent scholars that cannot afford such costs have no access. More re-
cently, in addition to a number of subject-specific indexes, other sources of general citation
data have been made available by other commercial companies, such as Google (Google
Scholar), Digital Science (Dimensions), and Microsoft (Microsoft Academic, formerly
Microsoft Academic Search). Sin embargo, although access to these is free, all have license re-
strictions on users’ ability to reuse and republish the citation data they provide, which limits
the full description and reproducibility of research studies using these data.
OpenCitations (http://opencitations.net) has been established for the specific purpose of dis-
rupting that status quo by providing a fully free and open alternative for accessing global schol-
arly citation data in Linked Data form (Bizer, Heath, & Berners-Lee, 2009). In this article, nosotros
introduce the main data and services that OpenCitations provides and describe the known
uses of its data within the scientometrics community and planned future developments in
terms of new data and initiatives.
An additional excellent source of open citation data, albeit limited to the biomedical do-
principal, is the recently released NIH Open Citation Collection, published by the Office for
Portfolio Analysis of the Office of the Director of the U.S. Institutos Nacionales de Salud
(Hutchins et al., 2019). Citations from this resource, which are specifically citations between
articles indexed in PubMed, can be retrieved using the iCite web service at https://icite.od.
nih.gov.
2. FROM THE ORIGINS TO THE INITIATIVE FOR OPEN CITATIONS
The first data set released that contained open bibliographic and citation data was the
OpenCitations Corpus (OCC), first made available in 2010 as the main output of the Open
Citations Project funded by the Joint Information Systems Committee ( JISC; Shotton, 2013b).
JISC (now Jisc, a UK not-for-profit company), was at that time a UK government-funded orga-
nization providing infrastructure services, such as JANET, a high-speed network for the UK
research and education community (https://www.jisc.ac.uk/janet), and awarding grants to sup-
port the development of innovative digital solutions for UK education and research. Sin embargo,
this initial version of the OCC was of limited scope. Between 2010 y 2016, a small number
of reference lists had also been made openly available by Crossref (https://crossref.org).
Sin embargo, the availability of open references from that source changed drastically in April
2017 as a result of the Initiative for Open Citations (I4OC; https://i4oc.org).
The I4OC—of which OpenCitations is one of the founding members, together with the
Wikimedia Foundation, Public Library of Science (PLoS), eVida, DataCite, and the Centre for
Culture and Technologies at Curtin University—was created to promote the release of open
citation data, specifically by asking scholarly publishers, who were already depositing the ref-
erence lists of their publications at Crossref, to make them open to everyone. Before I4OC
started, justo 1% of all the references deposited at Crossref by scholarly publishers were open.
Following discussions between I4OC and the major publishers, and other events publicizing
the Initiative over the past two years, más que 1,200 scholarly publishers (as of September
2019), including almost all the major scholarly publishers, have chosen to open their depos-
ited references at Crossref. Como resultado, the percentage of papers with references deposited at
Crossref for which the references are open has risen from 1% a 59%, and there are now over
500 million references openly available via the Crossref API. These major publishers include
the American Geophysical Union, the Association for Computing Machinery, BMJ, Cambridge
Estudios de ciencias cuantitativas
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OpenCitations
Prensa universitaria, Cold Spring Harbor Laboratory Press, EMBO Press, IOP Publishing, CON
Prensa, prensa de la Universidad de Oxford, the Royal Society of Chemistry, SAGE Publishing, Saltador
Naturaleza, taylor & Francisco, and Wiley.
The Initiative has also gathered an impressive group of important and supportive stake-
holders, including libraries (including the California Digital Library and British Library), estafa-
sortia (including the Open Research Funders Group and Open Access Scholarly Publishers
Asociación), projects (including CORE and OpenAIRE), organizaciones (including Mozilla
and the International Society for Scientometrics and Informetrics), companies (incluido
Microsoft Research and Figshare), y, En particular, funders (including the Bill & Melinda
Gates Foundation and Wellcome Trust) (Shotton, 2018). Además, several international
events, including the 2018 Workshop on Open Citations (https://workshop-oc.github.io) y
WikiCite 2018 (https://meta.wikimedia.org/wiki/ WikiCite_2018), have been held to promote
the open availability of citation data.
These open Crossref references have been the basis for the creation and/or extension of
commercial services, such as ScienceOpen (https://www.scienceopen.com) and Dimensions
(https://www.dimensions.ai), and OpenCitations has also used them to populate COCI, el
OpenCitations Index of Crossref Open DOI-to-DOI Citations, described below. Además,
several organizations are continuously acting to increase the support and provision of open
citation data. Por ejemplo, the Open Access Scholarly Publishers Association (OASPA) tiene
recently introduced the requirement that its members who are depositing reference lists with
Crossref must make these reference lists openly available, in line with I4OC recommendations
(see https://oaspa.org/oaspa-and-the-initiative-for-open-citations/).
3. THE BENEFITS OF OPEN SCHOLARSHIP AND OPEN CITATIONS
The Initiative for Open Citations is just one of the disruptive changes that are presently char-
acterizing the scholarly digital publishing landscape. As the open scholarship model gains
traction, subscription models for access to journal content are crumbling, as evidenced by
the Big Deal Cancellation Tracking website maintained by SPARC (https://sparcopen.org/
our-work/big-deal-cancellation-tracking/), which presently lists in excess of fifty academic li-
braries, institutions, and consortia that have cancelled their subscriptions to the journals of
Elsevier and other major publishers, judging that they no longer offer value for the money.
The past two decades have seen an explosive growth of open scholarly information
sources, providing radical alternatives to commercial sources of scholarly information.
Predicated upon the belief that all the fruits of publicly funded scholarly endeavor should
be openly available to all scholars and the general public, and complementing the open pub-
lication of knowledge more generally by Wikipedia (https://www.wikipedia.org/) and similar
sources, the open scholarship (Open Science) movement can be characterized as having four
distinct phases:
(cid:129) Open source software, of which Linux as an operating system (https://www.linux.com/)
and the Free Software Foundation (https://www.fsf.org/) are good examples.
(cid:129) Open access publication of scientific articles and the rise of open access publishers,
among which PLoS (https://www.plos.org/), eVida (https://elifesciences.org/), y
F1000 Research (https://f1000research.com/) are prominent.
(cid:129) Open research data sets, held in repositories, of which the Protein Databank (https://
www.wwpdb.org/) and the Dryad Data Repository (https://datadryad.org/) are just two
prime examples among many in the biological realm, and espoused by organizations
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OpenCitations
such as CODATA (http://www.codata.org/), the Research Data Alliance (https://www.rd-
alliance.org/), and the European Open Science Cloud (EOSC, https://www.eosc-portal.
eu/).
(cid:129) Open bibliographic metadata, held by scholarly infrastructure organizations, como
PubMed (https://www.ncbi.nlm.nih.gov/pubmed) and Crossref (https://www.crossref.
org/), whose services are invaluable to scholars worldwide.
Bibliographic metadata, the information describing scholarly publications, are key compo-
nents of the open scholarly ecosystem, and prime among these are bibliographic citations,
described in the introduction. Bibliographic citations are factual in nature, and such facts can-
not be copyrighted and should not be placed behind subscription paywalls.
Those that benefit from open citations include the following:
(cid:129) Researchers and authors, particularly those who are not members of the elite club of
research universities that can afford subscription access to the commercial citation in-
dexes WoS and Scopus.
(cid:129) Bibliometricians, who will be able to publish the research data upon which their re-
search findings are based.
(cid:129) Librarians, who will be better able to support their stakeholder communities (autores,
investigadores, estudiantes, institutional administrators) by providing free access to citations.
(cid:129) Funders, who can better assess the impact of scientific work and decide which re-
searchers, ideas, and projects are worth funding.
(cid:129) Academic administrators of research institutes and universities, who will more easily
be able to track the scholarly productivity and influence of their members.
(cid:129) Research managers, who will have open citation data available for integration in their
CRIS systems, including those using the Common European Research Information
Framework (CERIF).
(cid:129) Data repositories, which will benefit from open bibliographic citations between their
data sets and the articles describing them.
(cid:129) Publishers, who will have more readers guided from open citation data to their online
journals, which will secondarily attract additional article submissions.
(cid:129) Finalmente, computer scientists and software providers, who will be able to exploit this free
availability of citation data to build new applications and visualizations that we cannot
even begin to imagine.
4. A BRIEF INTRODUCTION TO SEMANTIC WEB TECHNOLOGIES
The change toward open scholarship described in the previous section is not only social but
also technological. Recientemente, one of the most discussed issues has been how to create a scal-
able infrastructure that can store and serve all the bibliographic and citation data of interest. A
centralized solution (es decir., having all the information stored in a unique database) is not feasible
in the long term, as the amount of data to be handled increases, due to performance issues and
physical space needed for managing a service (access, REST API, query) adecuadamente. El
Confederation of Open Access Repositories (COAR) has recently discussed this specific aspect
in its 2017 informe (COAR WG Next Generation Repositories, 2017). It suggests that there is an
urgent need for a “distributed globally networked infrastructure for scholarly communication,"
which should enable “cross-repository connections” by using “bidirectional links,” and that
should provide its data using formats that are “machine-friendly, enabling the development
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OpenCitations
of a wider range of global repository services.” Semantic Web technologies are one of the
strongest options for implementing such a globally distributed scholarly infrastructure.
The World Wide Web Consortium ( W3C; https://w3.org) has designed a suite of Semantic
Web technologies and standards (https://www.w3.org/standards/semanticweb/) that facilitate
the use of precise semantics for the encoding and creation of machine-processable data on
the web.
The main characteristics of the Semantic Web are simple:
(cid:129) All entities (p.ej., this article and its authors), their types (p.ej., the classes to which all
documents and people belong, such as “journal article,” “author”), and their relation-
buques (properties defining attributes or linking entities and their types together according
to a particular semantics, such as the fact that this article is authored by Silvio Peroni
and David Shotton), are identified by HTTP URIs (https://en.wikipedia.org/wiki/
Uniform_Resource_Identifier), and their related information can be retrieved from the
web by using such URIs.
(cid:129) Publicly available and commonly accepted structured vocabularies (ontologies, https://en.
wikipedia.org/wiki/Ontology_(information_science) are made available on the web to de-
fine the meanings of the different classes of entities and the relationships that link them.
(cid:129) The abstract data model used to define the information related to such entities, types,
and relationships is the Resource Description Framework (RDF) (Cyganiak, Wood, &
Krötzsch, 2014).
(cid:129) Each statement related to a particular entity is expressed in RDF as a subject–predicate–
object “triple”: Por ejemplo,
purl.org/spar/cito) is the ontology in which the property “cites” is defined.
(cid:129) RDF statements stored in different sources can be combined into interconnected infor-
mation networks (directed graphs, https://en.wikipedia.org/wiki/Directed_graph) forma-
ing “linked data”—thereby creating a web of knowledge, the Semantic Web, en
which the truth content of each original statement is maintained.
(cid:129) RDF triples can be stored in a particular type of database designed for graph data, conocido
as a triplestore (https://en.wikipedia.org/wiki/Triplestore).
(cid:129) Such data can be queried using SPARQL (harris & Seaborne, 2013), a special query
language able to retrieve and manipulate data stored in RDF format.
Further explanations of the Semantic Web and its applications to bibliographic information
are provided in a series of short didactic posts entitled “Libraries and linked data,” found on
our Semantic Publishing blog (Shotton, 2013a).
5. OPENCITATIONS: TOOLS AND SERVICES
OpenCitations is an independent infrastructure organization for open scholarship dedicated to
the publication of open bibliographic and citation data by the use of Semantic Web (Linked
Datos) tecnologías. It is also engaged in advocacy for open citations, particularly in its role as
a key founding member of I4OC. For administrative convenience, OpenCitations is managed
by the separate newly formed Research Centre for Open Scholarly Metadata at the University
of Bologna (https://openscholarlymetadata.org), of which the two authors of this article, Silvio
Peroni and David Shotton, are director and associate director, respectivamente.
OpenCitations espouses fully the founding principles of Open Science. It complies with the
FAIR data principles (Wilkinson et al., 2016) proposed by Force11 (https://www.force11.org)
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OpenCitations
that data should be findable, accessible, interoperable, and reusable, and it complies with the
recommendations of I4OC that citation data in particular should be structured, separable, y
abierto. On the latter topic, OpenCitations has recently published a formal definition of an Open
Citación (peroni & Shotton, 2018a) and launched a system for globally unique and persistent
identifiers (PIDs) for bibliographic citations—Open Citation Identifiers (OCIs).
5.1. Open Citation Identifiers
The OCI is a globally unique PID for open bibliographic citations, as defined by Peroni &
Shotton (2018a). The OCI system has been developed by OpenCitations, which maintains a
resolution service for OCIs at http://opencitations.net/oci. Given a valid OCI as input, this res-
olution service is able to retrieve metadata about the identified citation in RDF (as RDF/ XML
Turtle or JSON-LD), Scholix, JSON, or CSV formats.
Each OCI has a simple structure: the lowercase letters “oci” followed by a colon, seguido
by two sequences of numerals separated by a dash. The first sequence identifies the citing
publication and the supplier database within which its metadata are to be found (p.ej.,
Crossref or the OCC), and the second sequence identifies the cited publication and the sup-
plier database within which its metadata are to be found. Each supplier database is defined by
a numerical prefix assigned by OpenCitations containing a short sequence of positive integers
delimited by zeros: Por ejemplo, 020 is the Crossref prefix, y 030 is the OCC prefix.
Thus oci:0302544384-0307295288 is a valid OCI for a citation between two publications
recorded in the OCC with internal OCC identifiers 2544384 y 7295288 respectivamente, y
oci:02001010806360107050663080702026306630509-02001010806360107050663080
702026305630301 is a valid OCI for a citation between two publications recorded in Crossref
with the DOIs 10.1186/1756-8722-6-59 y 10.1186/1756-8722-5-31, respectivamente. Details
of how alphanumeric identifiers, such as DOIs, are transformed into the purely numerical se-
quences used in the OCIs are given by Peroni & Shotton (2019). OCIs are designed for ma-
chine processing rather than human readability, so the length of the numerical strings created
by such alphanumeric transformations is not a problem.
There are three main advantages of treating citations as first-class data entities in their own
bien, with their own globally unique PIDs:
1. All the information regarding each citation can be stored in one place, with associated
metadata.
2. Citations become easier to describe, distinguish, count, and process.
3.
If available in aggregate, citations described in this manner are easier to analyze using
bibliometric methods, such as to visualize citation networks or to determine how cita-
tion time spans vary by discipline.
It should be noted that OCIs are not opaque identifiers, as they explicitly encode directional
relationships between identified citing and cited entities, the provenance of the citation (es decir.,
the database that contains it), and the type of identifiers used in that database to identify the
citing and cited entities. The OCIs defining citations between a group of publications that cite
one another thus contain all the information required to construct the citation network of these
publicaciones.
OCIs have been accepted by the community, being recognized as PIDs for citations by the
EU FREYA Project (https://www.project-freya.eu) (Ferguson et al., 2018) and registered by
Identifiers.org (https://identifiers.org/oci), which is a PID registry and metaresolver.
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OpenCitations
5.2. The OpenCitations Data Model and the SPAR Ontologies
To enable the description of bibliographic and citation information in machine-readable
terms, OpenCitations provided, maintains, and updates the OpenCitations Data Model
(OCDM) (Daquino, peroni, & Shotton, 2019), briefly summarized in Figure 1.
The OCDM is used to model all the bibliographic and citation entities (es decir., the yellow rect-
angles in Figure 1, defining the classes of objects the data model allows one to describe), su
atributos (es decir., the green arrows), and the relations to other entities (es decir., the blue arrows). Todo
these aspects are exposed in any OpenCitation data set in RDF, using the “language” of the
Web semántica, in particular by employing OpenCitations’ SPAR Ontologies (http://www.
sparontologies.net) (peroni & Shotton, 2018b). Such usage permits the publication of biblio-
graphic and citation data as Linked Open Data (LOD), thereby conferring machine readability
and interoperability of the data on the web. The OCDM may also be employed by third parties,
either for their own use or to structure their data for submission to and publication by
OpenCitations.
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Cifra 1. A Graffoo diagram summarizing the OCDM, implemented using the OpenCitations Ontology (OCO) disponible en https://w3id.org/
oc/ontology.
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The OCDM allows one to record information about
(cid:129) published bibliographic resources (class fabio:Expression in Figure 1) that either
cite or are cited by another published bibliographic resources or that contain citing/
cited entities (p.ej., a journal containing an article or a book containing chapter);
(cid:129) possible resource embodiments (class fabio:Manifestation in Figure 1) defining
the particular physical or digital format in which a bibliographic resource was made
disponible;
(cid:129) bibliographic references (class biro:BibliographicReference in Figure 1), usu-
ally occurring in the reference list (and usually denoted by one or more in-text reference
pointers within a citing bibliographic resource) of a citing entity, that references another
bibliographic resource;
(cid:129) responsible agents (class foaf:Agent in Figure 1), such as people or organizations,
having a certain role with respect to a bibliographic resource (p.ej., an author of a paper
or book or the publisher of a journal);
(cid:129) the roles (class pro:RoleInTime in Figure 1) held by an agent with respect to biblio-
graphic resources (p.ej., a person being both the author of an article and the editor of a
libro);
(cid:129) the citations (class cito:Citation in Figure 1) between pairs of bibliographic re-
sources; y
(cid:129) the external identifiers (class datacite:Identifier in Figure 1), such as DOI,
ORCID, PubMedID, OCIs, associated with the bibliographic entities.
En noviembre 2019, a new release of the OCDM was published, revised and extended with
additional kinds of entities that enable the description of in-text reference pointers (class c4o:
InTextRefefencePointer in Figure 1) denoting bibliographic references—that is, the tex-
tual devices (p.ej., “[1]” or “Peroni & Shotton 2019”) that are embedded in the text of a doc-
ument within the context of a particular sentence, párrafo, or section (which are kinds of
discourse elements, defined by the class deo:DiscourseElement in Figure 1)—and the
citations they instantiate (linked via annotations, defined by the class oa:Annotation in
Cifra 1), accompanied by a description of their functions (es decir., the reason why a bibliographic
resource is cited; Teufel, Siddharthan, & Tidhar, 2006).
5.3. The OpenCitations Data Sets
In terms of data, OpenCitations first developed the OCC (http://opencitations.net/corpus;
(peroni, Shotton, & Vitali, 2017), a database of open downloadable bibliographic and citation
data recorded in RDF and released under a Creative Commons CC0 public domain waiver
(https://creativecommons.org/publicdomain/zero/1.0/), which currently contains information
about ∼14 million citation links to over 7.5 million cited resources. The current content of
the OCC has been mainly derived from biomedical articles within the Open Access Subset
of PubMed Central (https://www.ncbi.nlm.nih.gov/pmc/tools/openftlist/), harvested using the
Europe PubMed Central REST API (https://europepmc.org/RestfulWebService), and contains
information described following the OCDM.
In addition and separately, OpenCitations is currently developing a number of Open
Citation Indexes (http://opencitations.net/index), using the data openly available in third-party
bibliographic databases. The first and largest of these is COCI (http://opencitations.net/index/
coci; Heibi, peroni, & Shotton, 2019a), which presently contains information encoded in RDF
on more than 624 million citations, released under a CC0 waiver.
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The most recent index is the Crowdsourced Open Citation Index (CROCI; http://opencitations.
net/index/croci; Heibi, peroni & Shotton, 2019b), which is designed to host citation data submitted
by third parties—authors, editores, scholars—allowing them to upload to OpenCitations citation
data that are not elsewhere openly available, such as from their own publications, journals, y
reference collections, in an effort to fill the gap of missing citations from some publishers (particular-
larly Elsevier) that are not presently available at Crossref as open material.
The indexes contain information about the citations themselves, in which the citations, en-
stead of being considered as simple links, are treated as first-class data entities in their own
bien. This permits us to endow each citation with descriptive properties, such as the date on
which the citation was created, its time span (es decir., the interval between the publication date of
the cited entity and the publication date of the citing entity), and its type (p.ej., whether or not it
is a self-citation). An in-depth description of the definition and use of citations as first-class data
entities is provided by Shotton (2018). In contrast to the OCC, these indexes do not store meta-
data about the citing and cited bibliographic entities internally. Bastante, these entities are iden-
tified in the indexes by their unique identifiers (p.ej., DOIs), enabling bibliographic information
to be retrieved on the fly upon request by means of the related API (see the operation “meta-
data” at https://w3id.org/oc/index/api/v1 for additional information).
5.4. Provenance Information
The provenance of a certain resource concerns “the people, institutions, entidades, and activities
involved in producing, influencing, or delivering a piece of data or a thing” and “is crucial in
deciding whether information is to be trusted, how it should be integrated with other diverse
information sources, and how to give credit to its originators when reusing it” (Moreau &
Missier, 2013).
All the entities (bibliographic resources, citas, agents involved in the publication pro-
impuesto, such as authors and publishers) included in the data sets released by OpenCitations are
accompanied by provenance information, so as to keep track of the curatorial activities related
to each entity, the curatorial agents involved, and the sources used to obtain such data. En
addition, OpenCitations also tracks how the data related to its entities may have changed in
tiempo, to allow one to reconstruct the particular description status (or snapshot) of an entity at a
specified time. This has been technically implemented by extending the Provenance Ontology
(Lebo, Sahoo, & McGuinness, 2013) with a SPARQL-based construct that has been inspired by
existing works on change tracking mechanisms in documents created through word proces-
sors, such as Microsoft Word and OpenOffice Writer (peroni, Shotton, & Vitali, 2016).
5.5. OpenCitations Software
All the aforementioned data are made available in RDF (Cyganiak, Wood, & Krötzsch, 2014),
this being the main format for expressing structured machine-readable data for the web, y
are stored in specialized graph databases for RDF known as triplestores. OpenCitations pro-
vides open source software of generic applicability for searching, browsing, and querying its
bibliographic and citation data (https://github.com/opencitations). De este modo, programmatic access
to the OCC and COCI triplestores may be obtained using queries in SPARQL, the RDF query
idioma (harris & Seaborne, 2013), via their SPARQL endpoints, in JSON and CSV formats by
using the OpenCitations REST API created using RAMOSE (https://github.com/opencitations/
ramose), OpenCitations’ application for creating REST APIs over SPARQL endpoints (Heibi,
peroni, & Shotton, 2019C), or via HTTP requests in different formats (HTML, RDF/ XML,
Turtle, or JSON-LD, via content negotiation). The OpenCitations data sets can also be explored
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by humans using OSCAR, the OpenCitations RDF Search Application (https://github.com/
opencitations/oscar), employing author name, work title (or part thereof ), or identifier (DOI,
ORCID) as input, and the returned results can then be browsed using OSCAR’s associated
OpenCitations RDF Resource Browser (https://github.com/opencitations/lucinda), LUCINDA.
Metadata for individual bibliographic entities within OCC and citations within COCI can also be
accessed via a simple web form using their individual URIs (p.ej., https://w3id.org/oc/corpus/br/1),
and downloads of the entire OCC and COCI data sets are possible from dumps made periodically
and stored on Figshare (http://opencitations.net/download), so as to support large-scale sciento-
metric analyses using the whole content of the data sets.
6. THE SUSTAINABILITY OF OPENCITATIONS
To build an infrastructure that is sustainable in the long term and follows pure Open Science prin-
ciples, OpenCitations has adopted in full the Principles for Open Scholarly Infrastructures (Bilder,
lin, & Neylon, 2015), which recommend three sets of principles to which open scholarly infra-
structures should adhere, under the headings Insurance, Gobernancia, and Sustainability.
6.1.
Insurance
OpenCitations completely fulfills the requirements of Bilder, lin, & Neylon (2015) designed to
ensure that the work of OpenCitations would survive should OpenCitations itself cease to ex-
ist. All the software released by OpenCitations is open, available on GitHub at https://github.
com/opencitations and released with the ISC License (https://choosealicense.com/licenses/isc/),
which is a permissive free software license that allows maximum reuse of the software in differ-
ent contexts, commercial or noncommercial. All the models (es decir., the OCDM and the SPAR
Ontologies) used to describe the bibliographic and citation data provided OpenCitations are
made available using a CC-BY license (https://creativecommons.org/licenses/by/4.0/), mientras
all the OpenCitations data are released using the CC0 waiver so as to maximize their reuse, y
are obtainable by a variety of means, as described above.
En efecto, as all OpenCitations software and data are open and recorded using open stan-
dards, it is possible even now for third parties to take and reuse the data, or migrate it to
new platforms, at any time. Finalmente, it is worth mentioning that OpenCitations does not hold
nor will it seek to obtain a patent for any of its products.
6.2. Gobernancia
OpenCitations is engaged with citation data covering the whole spectrum of the scholarly re-
search domain. Además, all the OpenCitations applications—such as OSCAR, LUCINDA,
and RAMOSE (https://github.com/opencitations/ramose) used to develop the aforementioned
REST APIs—have been designed to be of generic usefulness and are made available in a man-
ner that permits their reuse by members of the community in a plethora of different scenarios
that need not be related in any way to bibliographic and citation data.
OpenCitations, as an independent infrastructure organization for open scholarship, is current-
ly directed by its two directors, the authors of this article. For administrative convenience, es
managed by the Research Centre for Open Scholarly Metadata, an independent research center
within the University of Bologna, which is itself, as a public Italian university, a nonprofit insti-
tution. The Research Centre has an International Board drawn from leaders within the main bib-
liographic stakeholder communities of relevance (librarians, bibliometricians, academics, datos
service providers, etc.) who have shown past solid commitment to open scholarship. The stat-
utes of the Research Centre will ensure that OpenCitations’ original aim of free provision of open
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bibliographic and citation data, services, and software is maintained and that OpenCitations as
an organization cannot in future be taken over or controlled by commercial interests nor be-
come involved in political, regulatory, legislative, or financial lobbying of any kind.
6.3. Sustainability
Hasta ahora, OpenCitations and its products have been funded by specific grants from different in-
vestors. En particular, the first OCC prototype was funded by a small one-year grant from JISC
en 2010, which was awarded a six-month extension. There then followed a period without
external funding, which ended with the award of a grant from the Alfred P. Sloan
Base (https://sloan.org) for the OpenCitations Enhancement Project (https://sloan.org/
grant-detail/8017), funding we have used to achieve the current status of OpenCitations.
We now have a new grant from the Wellcome Trust for a one-year project entitled “Open
Biomedical Citations in Context Corpus," (https://wellcome.ac.uk/funding/people-and-projects/
grants-awarded/open-biomedical-citations-context-corpus), that started on July 1, 2019.
OpenCitations will continue to apply for targeted grants for specific projects, either alone or
with partners, by participating in H2020 calls and approaching additional funders and orga-
nizations, including the Alfred P. Sloan Foundation, the Chan-Zuckerberg Initiative (https://
chanzuckerberg.com), and the Arcadia Fund (https://www.arcadiafund.org.uk).
Because all OpenCitations’ data and services are open, it has nothing to sell or against
which to charge membership fees. Además, OpenCitations’ basic philosophy that all its
data and services should be free mitigates against any form of “freemium” income generation.
Long-term sustainability for OpenCitations thus requires ongoing support from the scholarly
comunidad, perhaps following the SCOSS (http://scoss.org) and the Invest in Open
Infrastructure (https://investinopen.org) models of crowdsourced support from stakeholders
within the open scholarship community, or alternatively by “adoption” by one or more major
scholarly libraries, philanthropists, or academic funding agencies, with those institutions or
individuals thus gaining credit for supporting OpenCitations in conformity with their missions
and goals. OpenCitations is no different from any other open infrastructure organization that
provides free data services of value, such as PubMed, Europe PubMed Central, or the Protein
Data Bank.
We are therefore delighted to announce that OpenCitations has been selected by the
SCOSS board for its second round of crowdfunding support, as OpenCitations aligns well with
Open Science goals and is an innovative service; open citation data are important to the com-
munity because they could support change in research assessment, y, if successful, could be
a game changer by challenging established proprietary citation services. Starting in January
2020, SCOSS will invite research organizations, scholarly institutions, and funders of all sizes
throughout the world to contribute financially in proportion to their size and ability to sustain
OpenCitations’ operations over the next three years as it transitions into a global scholarly
infrastructure organization with a secure financial footing. As directors of OpenCitations,
we invite and encourage you to support OpenCitations through this SCOSS initiative.
If OpenCitations continues to expand its coverage of the scholarly domain as it has been
doing (from ∼14 million to over 624 million citations in the past two years), so as to offer a
genuine alternative in terms of coverage to the extensive citation data offered by WoS and
Scopus ( WoS has over a billion citations), then it stands every chance of attracting financial
support from university libraries and other scholarly institutions at a fraction of the cost of their
current subscriptions to those commercial citation indexes.
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7. USAGE STATISTICS
In the past year, the OpenCitations website, with all its services and pages, has been accessed
más que 3.1 million times by more than 68,000 unique visitors (identified by their IP ad-
dresses)—we have excluded from these counts all accesses made by automated agents and
bots. Específicamente, the number of accesses made between April 2018 and March 2019 (inclu-
sive) se muestra en la figura 2, listing five main categories of information access services available
in the website: the direct HTTP access to a particular bibliographic resource and/or citation
(“HTTP_CONT_NEG”), the search/browse interfaces (“INTERFACE”), the REST APIs (“API”),
SPARQL queries to the endpoints (“SPARQL”), and “other” (visits to the OpenCitations home-
page and other web pages). It is worth mentioning that the APIs were formally introduced in
Junio 2018, following a few internal experiments run in May 2018. They have rapidly become
the main service used for querying the citation data available in OpenCitations.
These data are complemented by the chart shown in Figure 3, which shows the number of re-
quests from different countries worldwide (identified by the IP addresses of the requests). As is clear
from the diagram, those countries making the most requests were Italy, Poland, y los estados unidos,
followed by Brazil, Francia, Los países bajos, and Spain, then China, Alemania, India, and the UK.
En figura 4, we show the statistics concerning the OpenCitations resources made available
on Figshare (es decir., the dumps of the data sets made available by OpenCitations, así como el
definition documents). In the past year, these Figshare documents have obtained more than
20,000 views and 3,000 downloads overall.
All the CSV data used to create the foregoing charts are available in OpenCitations (2019).
8. ADOPTION OF OPENCITATIONS BY THE COMMUNITY
The open citation data published by OpenCitations will benefit all scholars and researchers,
particularly those who are not members of the elite club of research universities that can afford
subscription access to WoS and Scopus. These data are of particular value to bibliometricians
because they not only permit open research but also allow republication of the actual data
Cifra 2. The overall number of accesses to the OpenCitations website pages and services in the past year, month by month.
“HTTP_CONT_NEG” (es decir., HTTP content negotiation) indicates direct access to stored resources by means of their HTTP URI, “INTERFACE”
indicates the use of web interfaces for browsing and searching bibliographic and citation data, “API” shows the calls to the various
OpenCitations REST APIs, “SPARQL” indicates the calls to the OpenCitations SPARQL endpoints, and “OTHER” lists the accesses made to
all the other resources. Note that the y-axis is logarithmic.
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Cifra 3. The map showing the relative frequency of accesses to the OpenCitations website in the last year, from April 2018 to March 2019,
organized by country. Only a few countries worldwide, colored in white, did not access the website in the past year.
upon which the research findings are based, thus enabling the reproducibility of bibliometrics
and scientometrics studies. This is rarely possible when the research is based on data from
proprietary citation indexes. Such scholars will now be able to pursue their studies with greater
freedom, following reference trails through the citation network without hindrance, and have
their own publications more easily found, discussed, and cited.
En años recientes, several researchers have already used the data provided by OpenCitations for
such studies. Por ejemplo, Kamińska (2017) published a case study showing the possibilities of
running bibliometric analysis on the open citation data of PLOS One articles available in the OCC.
COCI data, downloaded from the CSV dump available at http://opencitations.net/download, tener
also been used in at least three bibliometric studies recently. During the LIS Bibliometrics 2019
Cifra 4. The overall number of views and downloads to all the OpenCitations resources stored in
Figshare—mainly data set dumps and definition documents.
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Event, Pearson (2019) presented a study run on publications by scholars at the University of
Manchester that used COCI to retrieve citations between these publications to investigate pos-
sible cross-discipline and cross-department potential collaborations. Similarmente, COCI data
were used to conduct an experiment on the latest Italian Scientific Habilitation (the national
exercise that evaluates whether a scholar is appropriate to receive an associate/full professor-
ship position in an Italian university), which aimed at trying to replicate some of the outcomes
of this evaluation exercise for the Computer Science research field using only open scholarly
datos, including the citations available in COCI, rather than citation data from subscription
services (Di Iorio, peroni, & Poggi, 2019). Finalmente, COCI has also been used to explore the
roles of books in scholarly communication (Zhu et al., 2019).
OpenCitations data have also been used by several tools dedicated to the visualization of
citation graphs and other scholarly networks. VOSviewer (van Eck & waltman, 2010) (http://
www.vosviewer.com) is a software tool, developed at the Leiden University’s Centre for
Science and Technology Studies (CWTS), for constructing and visualizing bibliometric net-
obras, which may include journals, investigadores, or individual publications and may be con-
structed based on citation, bibliographic coupling, cocitation, and coauthorship relations.
Starting from version 1.6.10 (released on January 10, 2019), VOSviewer can now directly
use citation data stored in COCI, retrieved by means of the COCI REST API. Citation Gecko
(http://citationgecko.com) and OCI Graphe (https://dossier-ng.univ-st-etienne.fr/scd/www/oci/
OCI_graphe_accueil.html) are two other examples of web tools that allow one to map a re-
search citation network using some initial seed articles. Both of them use COCI data (accedido
via the REST API) to generate the citation network shown in the browser. An alternative visu-
alization tool is VisualBib (http://visualbib.uniud.it/; Corbatto & Dattolo, 2018), which uses the
data in the OCC, among others, to support researchers who wish to create, refine, and visu-
alize a bibliography from a small set of significant papers or a restricted number of authors.
In addition to the aforementioned activities, OpenCitations is currently collaborating with a
number of academic projects related to the management of bibliographic and citation data, a
both promote the use of the OCDM and provide a publication venue for the citation data that
these projects are liberating from the scholarly literature. Among these, it is particularly worth
mentioning the Venice Scholar Index (https://venicescholar.dhlab.epfl.ch), the Linked Open
Citations Database (LOC-DB; https:// locdb.bib.uni-mannheim.de; Lauscher et al., 2018),
and the EXCITE Project (http://excite.west.uni-koblenz.de; Hosseini et al., 2019).
9. CONCLUSIONS AND FUTURE DEVELOPMENTS
The main goal of OpenCitations is to provide open scholarly bibliographic and citation data
and related services to all possible users, to allow anyone to use such data for any purpose. So
far, OpenCitations has released more than 624 million citations and created several interfaces
for facilitating their consumption. Sin embargo, the plan for the next couple of years is to expand
the existing citation data available, as well as to create new data sets to serve additional needs.
En particular, following the success of COCI, OpenCitations will release the following new indexes
of existing open citation data sets: the OpenCitations Index of open Wikidata citations (WOCI), el
OpenCitations Index of open DataCite citations (DOCI), and the OpenCitations Index of open
citations within the Dryad Data Repository (DROCI). This will allow OpenCitations to extend
hugely the breadth of coverage of citation data available in its indexes.
In a major new initiative to be undertaken in collaboration with experts in bibliometrics and
cienciometría, OpenCitations will next develop OpenCitations Meta, a new database contain-
ing additional metadata relating to scholarly publications, which will include many of the kinds
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OpenCitations
of information described in the OCDM but currently lacking in the OCC, specifically including
publications’ abstracts, keywords, author affiliations, and funding details, information of crucial
value for informed bibliometric analyses of research. This information will be of particular inter-
est to universities and research institutes wishing to evaluate the output of their own scholars. En
addition, this new database has a crucial role for improving the existing OpenCitations APIs,
because it would allow more complex calls on them. It will also reduce the waiting time pres-
ently experienced by users while our systems pause for responses from external API services
(p.ej., the Crossref API [https://api.crossref.org]) to retrieve the metadata of the bibliographic re-
sources involved in a citation.
By storing such extended bibliographic metadata “in house” in OpenCitations Meta, we will
be able to offer a faster and richer service. Además, it will avoid duplication of data by ef-
ficiently permitting us to keep in the Meta database a single copy of the metadata for each of the
bibliographic entities involved as citing or cited entities in the different OpenCitations’ citation
indexes, because these same citing and cited entities may be referenced independently within
the different indexes, from Crossref references, Wikidata references, DataCite references, etc..
These developments should be understood as a radical refocusing of OpenCitation’s data pro-
vision strategy. Initially, the OCC was conceived as a single database that would contain all our
bibliographic and citation information. Ahora, with the developments of (a) the OpenCitations
Indexes containing citation data but not metadata about the citing and cited bibliographic entities,
(b) the proposed OpenCitations Meta database that will contain bibliographic metadata but not
citation data, y (C) the new OpenCitations database to be developed as part of our funded
Wellcome Trust project that will house textual fragments that constitute the citation contexts of
each in-text citation occurrence—see Peroni (2018) for additional information—we are mov-
ing to a federated system of interoperable and complementary OpenCitations triplestore
resources—following the guidelines in COAR WG Next Generation Repositories (2017).
This is because, as OpenCitations’ coverage of the global citation landscape expands, it will
become technically inappropriate to handle and maintain everything (metadata, citas, ref-
erence lists, in-text reference contexts, abstracts, etc.) within a single repository. Better to or-
ganize each specific data type within one of a set of complementary and interoperable
repositorios, each encoding data in RDF according to the (expanded) OCDM and all search-
able using federated SPARQL queries.
This segregation of distinct data types into different triplestore repositories can then, in fu-
tura, be extended. And these different repositories can, if necessary, be maintained on different
computers at different locations across the internet or in the cloud, equipped with different
hardware according to the specific needs of each repository. General interoperability will
be guaranteed by means of SPARQL and its federated service for queries, by using the same
generic data model (es decir., the OCDM) and employing standard web and Semantic Web proto-
cols and standards for describing all these data.
In future, the OpenCitations “New Corpus” will thus be a set of federated SPARQL-based
and OCDM-encoded repositories, each describing a specific type of data, that can talk with
entre sí.
Our plan is to continue to use the original OCC database itself as a kind of experimental
sandbox (https://en.wikipedia.org/wiki/Sandbox_(software_development)) in which all the
data types handled by OpenCitations can be stored together and over which we can test
new software and new extensions to the OCDM on a known large but finite set of meaningful
papers and their references harvested primarily from the OA subset of Europe PubMed Central.
Estudios de ciencias cuantitativas
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OpenCitations
This approach of using a set of complementary and interoperable repositories searchable
using federated SPARQL queries opens the possibility of wider information federation between
those resources maintained by OpenCitations and similar interoperable open resources main-
tained by third parties. Those might provide, Por ejemplo, information about the publication
types of journal items (research articles, comment and opinion pieces, corrigenda, reviews,
etc.) in one repository that might be maintained by Crossref; autores, their ORCID and/or
the Virtual International Authority File ( VIAF, https://viaf.org) identifiers and current and past
institutional affiliations in another repository, ideally maintained by ORCID and VIAF them-
selves; or the geographical focus of published infectious disease reports in yet a third, possibly
maintained by WHO. Those resources would be maintained by third parties, thereby spread-
ing the load of providing open scholarly information, and what would unite them would be
their use of Semantic Web technologies, SPARQL, and the common data model.
COMPETING INTEREST
The authors are the directors of OpenCitations, the subject of this paper.
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