OpenCitations’ 2022: one year, numerous outcomes

2023 has arrived, and it’s the time to put aside the old agendas and to fill the freshly printed organizers with new tasks. However, it is also worth looking back over the achievements of the past year.

So, before consigning 2022 to the archives, we want to take a moment to recall and celebrate all that OpenCitations has achieved over the past year, in terms of technical developments, community building, and our own internal organization.

Clarifying our objectives

Since its foundation, OpenCitations has been presented to the scholarly community as a infrastructure organization guided by the values of openness, equity and accessibility. In 2022, we publicly clarified our mission, our aims and our unique points by publishing these three documents:

In addition, we continued to monitor OpenCitations’ compliance with the Principles of Open Scholarly Infrastructure. Because one area in which we were still falling short involved stakeholder involvement in OpenCitations’ governance, in 2022 OpenCitations started reflecting on its organizational structure by creating a Governance Evolution Working Group. In addition, we explored the further development of our stakeholder community by establishing a parallel Community Building Working Group. These two small working groups, involving members of the OpenCitations International Advisory Board, will continue to meet throughout 2023 to develop strategic plans concerning OpenCitations open governance structure and outreach.

More people, more tools

In 2022, OpenCitations welcomed two new full-time developers to its team: Arianna Moretti as software developer, and Ivan Heibi as the person responsible for OpenCitations’ technical infrastructure. As of January 2023, the OpenCitations team consists of our two directors Silvio Peroni and David Shotton, our administrator and research manager Claudio Fabbri, myself as our communications director and community development manager, and our four developers: Giuseppe Grieco (software and systems developer), Arcangelo Massari (developer of OpenCitations Meta), Arianna Moretti (software and indexes developer), and Ivan Heibi (technical infrastructure manager). This team meets weekly at the Research Centre for Open Scholarly Metadata within the University of Bologna.

This enlarged technical workforce has been crucial in the development of the two new OpenCitations indexes, DOCI (citations from DataCite) and POCI (citations from PubMed), and our new database of bibliographic metadata, OpenCitations Meta, all of which were released in December 2022:

  • DOCI, the OpenCitations Index of DataCite open DOI-to-DOI citations, is an RDF dataset containing details of all the citations that are specified in the most recent dump of DataCite metadata. The citations available in DOCI are treated as first-class data entities, with accompanying properties including the citations timespan, modelled according to the OpenCitations Data Model. Its first release (December 2022) contains information on 169,822,752 citations from 358,277 bibliographic resources.
  • POCI, the OpenCitations Index of PubMed open PMID-to-PMID citations, is an RDF dataset containing details of all the citations from publications bearing PubMed Identifiers (PMIDs) to other PMID-identified publications, harvested from the National Institutes of Health Open Citations Collection (NIH-OCC). As for DOCI, the citations available in POCI are treated as first-class data entities, with accompanying properties including the citations timespan, modelled according to the OpenCitations Data Model. Its first release contains 717,654,703 citations from 26,024,862 bibliographic resources.
  • OpenCitations Meta database stores and delivers bibliographic metadata for all the publications involved as citing or cited publications in the citations recorded in the OpenCitations Indexes. For each publication, the metadata exposed by OpenCitations Meta includes the publication’s title, type, venue (e.g. journal name), volume number, issue number, page numbers, publication date, and identifiers such as Digital Object Identifiers (DOIs) and PubMed Identifiers (PMIDs). In addition, OpenCitations Meta includes details of the main actors involved in the document’s publication, i.e., the names of the authors, editors, and publishers, each with its own additional metadata and identifier (e.g. ORCID). Its first release (December 2022) contains information on 87,321,593 bibliographic entities, 277,750,235 authors and 2,359,301 editors (counted by their roles, without disambiguating individuals), 710,226 publication venues, and 17,268 publishers.

As can be seen on OpenCitations roadmap, we have many tasks already scheduled for 2023, and for this reason OpenCitations team needs to expand further: we now seek applicants for a new Research Fellow position to work on the further development of the OpenCitations technical infrastructure from April 2023.

Our role in the community

Throughout 2022, the OpenCitations team has participated both online and in person in a number of international events, presenting details of OpenCitations to potential stakeholders and to Open Science experts, and connecting with similar organizations.

In October, OpenCitations organized our third workshop, the Workshop on Open Citations and Open Scholarly Metadata 2023, as a three-hour online event, to gather together and connect researchers, publishers and policy makers interested in the creation, reuse and improvement of open citation data and open scholarly metadata. The invited speakers provided a broad overview of the current state and future development of open bibliographic metadata and the scholarly record, and the role of OpenCitations in research assessment.

We have participated in two major European research and development project: In the context of the EC-funded project RISIS2 we released the new database OpenCitations Meta and the index DOCI, while within the OpenAIRE-Nexus project network we continue to provide open bibliographic citations as part of the open data components of OpenAIRE and the European Open Science Cloud. During 2022, the OpenAIRE team supported our activities in many areas, with a particular attention to communication and dissemination, as we explained in our blog post OpenCitations and EC funding: OpenAIRE Nexus and RISIS2.

The end of 2022 brought with it two new exciting collaborations that will be developed during the coming year: the inclusion of OpenCitations in the Jisc Collections, thereby permitting UK institutions to support OpenCitations through JISC; and our involvement in the GraspOS project (starting date January 1, 2023) as an open and trusted federated infrastructure for next-generation research metrics and indicators.

Continuing support

At OpenCitations we are deeply grateful to all our members and donors that have supported us financially so far, thus making it possible to enlarge our team, maintain our technical infrastructure, and work on our new data releases.

Much of our success has been due to our selection as an infrastructure worthy for inclusion in the SCOSS second pledging round in 2019. SCOSS helped us get in touch with a wider community, and enabled OpenCitations to have an increasingly significant role in the research environment. OpenCitations is now part of the SCOSS family, the community of all SCOSS-supported infrastructures, that provides a framework of mutual support for the the Infrastructures during their growth and a safety net should troubles occur along the path.

However, OpenCitations still has a long way to go along the path from being a sustainable open infrastructure to becoming a sustained open infrastructure providing reliable and invaluable services to our global stakeholder community. There are still many long-planned activities that we hope to initiate in the near future, given sufficient resources. This is why OpenCitations requires ongoing support from the scholarly community as our three-year period of SCOSS sponsorship draws to a close. All the information you may require to start helping us financially is available on the OpenCitations website:

2023 has just started. With your support, you can help us in making it an impactful year for OpenCitations and the Open Science community as a whole.

OpenCitations in Five Hundred Words

Yesterday I gave a lightning talk at the 2021 OASPA Conference, with the title OpenCitations – what does the future hold? The poster accompanying my talk, published on Zenodo at, is reproduced below.

Poster for 2021 OASPA Conference Lightning Talk

Here is what I said:

= = =

Most of the talks at this conference have focussed on open access to textual content. But open bibliographic metadata is also vitally important, not least to enable the calculation of metrics that are both open and reproducible.

OpenCitations is a not-for-profit open infrastructure that provides such free access to global scholarly citations. We hold dear the values and principles that underpin Open Science, and are early adopters of the Principles of Open Scholarly Infrastructure (POSI) and the FAIR data principles.

Our largest citation index is COCI, the OpenCitations Index of Crossref open DOI-to-DOI citations, which currently indexes approximately 1.2 billion citations, released to the public domain under a CC0 waiver.

Our goal is for OpenCitations to provide open bibliographic citation information having scope, depth, accuracy and provenance that surpasses that of the commercial citation indexes, for access to which scholarly institutions presently pay enormous annual subscriptions.

I wish to mention just two of our planned developments:

OpenCitations Meta is a new database that will enable us to store in-house full bibliographic metadata about citing and cited publications. This will have two advantages: It will speed user queries, since we will no longer have to wait for responses to on-the-fly API calls to Crossref and ORCID to retrieve such metadata. More importantly, it will enable us to index the large number of references involving publications that do not have DOIs, something that for technical reasons is presently lacking.

Additionally, we plan to develop new citation indexes over other sources of open references, starting with DOCI, indexing references from DataCite, and NOCI, indexing the content of the NIH Open Citation Collection.

Our progress has until recently been severely manpower-limited. However, OpenCitations was fortunate to have been selected by SCOSS, the Global Sustainability Coalition for Open Science Services, as an open infrastructure providing a unique and valuable service, and worthy of crowdfunded financial support by the global stakeholder community of research institutes, academic libraries, funders and publishers.

As a result of the generous support so far provided or pledged by ~50 such institutions, we have already reached about one-third of our requested SCOSS budget, enabling us this year to appoint new staff to start our planned technical developments, to support our community outreach, and to help take our vision forward.

Such financial support is vital for our sustainability, since we generate no income from our provision of free data, services and software. We thus invite you too to contribute to OpenCitations.

However, we also seek community involvement in other ways: participation in the community-led governance of OpenCitations; help in developing our open source software and services; curatorial involvement to improve OpenCitations data; and collaborations with other like-minded infrastructures to develop federated access to open scholarly information of all types, thereby returning control over such information to the academic community that generated it in the first place.

If you would like to work with OpenCitations in any of these ways, please contact me.

Cite this article as: davidshotton, "OpenCitations in Five Hundred Words," in OpenCitations blog, 24/09/2021,

Introducing InTRePIDs – In-Text Reference Pointer Identifiers


Readers of this blog will be familiar with Open Citation Identifiers (OCIs), described in an earlier post and formally defined in [1]. OCIs enable bibliographic citations, treated as first class information entities, to be uniquely identified and referenced, and are used to identify the >624 million individual citations indexed in the latest release of COCI, the OpenCitations Index of Crossref open DOI-to-DOI citations, as described in a recent post.

However, COCI and similar citation indexes do not provide any information about where within the citing paper a citation is generated, the textual contexts of the in-text reference pointers, or the reasons for including different in-text reference pointers denoting the same reference at different points within the text.

As explained in the preceding post describing the Open Biomedical Citations in Context Corpus funded by the Wellcome Trust and under development by OpenCitations, deep citation analysis requires a more nuanced approach to citations, which acknowledges that each in-text reference pointer that denotes a bibliographic reference in the reference list of a citing publication instantiates its own citation, as shown in Figure 1.

Figure 1. Citations between a citing paper and a cited paper instantiated both by the inclusion of a bibliographic reference within the reference list of the citing paper and by the inclusion within the text of the citing paper of one or more in-text reference pointers denoting that reference.

The pointer citations clearly involve the same cited publication as does the reference citation itself, but each has its own unique characteristics: the location and textual context of its in-text reference pointer within the text of the citing publication, and its particular rhetorical function which is determined by that context.

If the reference citation is open (as defined in [2]) and identified by an OCI, each in-text reference pointer related to that citation can be identified uniquely using an In-Text Reference Pointer Identifier (InTRePID).

InTRePIDs facilitate in-depth scholarship on in-text reference pointer locations and citation functions, and fine-grained analysis of the relationships between publications, by making it possible

  • to identify each in-text reference pointer with a unique PID,
  • to distinguish references that are cited only once from those that are cited multiple times,
  • to see which references are cited together (e.g. in the same sentence or within an in-text reference pointer list),
  • to determine from which section(s) of the article references are cited (e.g. Introduction, Methods, Discussion), and, potentially,
  • to determine the rhetorical function of the citations from analysis of their textual contexts, by the application of natural language processing, machine learning and artificial intelligence techniques to conduct sentiment analysis on the citation contexts.

Definition of an InTRePID

An InTRePID is composed of two parts separated by an oblique stroke



  • <oci-numerals> is the numerical part of the OCI uniquely identifying the particular open citation to which the in-text reference pointer and its denoted bibliographic reference relate. Thus an InTRePID can be assigned for any in-text reference pointer that relates to an open citation for which a valid OCI has been assigned;
  • <ordinal> identifies the nth occurrence of an in-text reference pointer within the text of the citing paper relating to that citation; and
  • <total> defines the total number of in-text reference pointers denoting that bibliographic reference within the citing paper.

For example, intrepid:070433-070475/46 is a valid InTRePID for an in-text reference pointer defined within the OpenCitations Citations in Context Corpus.

A formal definition document for the InTRePID is given in [3].

Exemplar in-text reference pointers

Consider the following citing paper:

Zou, J. et al. (2020). Phenotypic and genotypic correlates of penicillin susceptibility in nontoxigenic Corynebacterium diphtheriae, British Columbia, Canada, 2015–2018. Emerging Infectious Diseases, 26: 97-103.

This paper contains six in-text reference pointers denoting Reference 13 in the reference list:

13. Lowe, C. et al. (2011). Cutaneous diphtheria in the urban poor population of Vancouver, British Columbia, Canada: a 10-year review. J. Clinical Microbiology 49: 2664-2666.

The InTRePIDs for these pointers are recorded within the OpenCitations Biomedical Citations in Context Corpus, together with the corpus identifiers and DOIs of the citing and cited papers, as shown in the excerpt presented in Figure 2.

Figure 2. An excerpt from the OpenCitations Biomedical Citations in Context Corpus, showing highlighted the InTRePIDs for the six in-text reference pointers within Zou, J. et al. (2020) denoting Reference 13, the reference to Lowe, C. et al. (2011), together with the internal corpus identifiers for each in-text reference pointer, and the corpus identifiers and DOIs for the citing and cited papers.

Of these six in-text reference pointers, having InTRePIDs intrepid:070433-070475/1-6 to intrepid:070433-070475/6-6, the first and the fourth of these, together with their document locations, their embedding sentences, their in-text reference pointer lists, and their InTRePIDs, chosen as examples, are as follows:

Introduction. “Nontoxigenic strains have been shown to have epidemic potential, causing infections in persons afflicted by homelessness, alcohol abuse, and injection drug use (9,13–15).” (intrepid:070433-070475/1-6)

Discussion. “We also noted ST5 and ST32 in our review from downtown Vancouver during 1998–2007 (13).” (intrepid:070433-070475/4-6)

The first of these discusses those people most susceptible to diphtheria infection, while the other discusses which multilocus sequence types (STs) of C. diphtheriae were found, thus relating to the organism causing the infection rather than to the infected individuals. The rhetorical function of these two in-text reference pointers is quite distinct.

To permit this information to be recorded within the OpenCitations Citations in Context Corpus, extensions were required to the OpenCitations Data Model, a new extended version of which was recently published [4], as described in a related blog post.

The OpenCitations InTRePID Resolution Service

To support the use of InTRePIDs to identify in-text reference pointers, OpenCitations has recently developed an InTRePID Resolution Service (currently in ‘beta’ in its development cycle), which is running at A screenshot of this service is shown in Figure 3.

Figure 3. A screenshot of the user interface of the InTRePID Resolution Service.

In addition to using the Web user interface shown in Figure 3, InTRePIDs can be entered into this resolution service in the form of resolvable URIs, e.g.

As shown in Figure 4, the OpenCitations InTRePID Resolution service returns metadata concerning the in-text reference pointer identified by the InTRePID, and the bibliographic reference that it denotes, from which further information about the citation and the citing and cited publications may be obtained by following the links provided.

Figure 4. A screenshot of the Web page displaying metadata returned by the InTRePID Resolution Service.

Note that as well as rendering this information in HTML on a web page, the resolution service can also provide it in a variety of machine-readable formats.


InTRePIDs, which enable the identification of individual in-text reference pointers, and the InTRePID Resolution Service, are new services from OpenCitations that will facilitate scholarship on the textual contexts and rhetorical functions of such in-text reference pointers, and of the citations that they instantiate.

InTRePIDs were first announced on 30th January 2020 at PIDapalooza 2020 in Lisbon, the Open Festival of Persistent Identifiers.


[1] Silvio Peroni and David Shotton (2019): Open Citation Identifier: Definition. Figshare.

[2] Silvio Peroni and David Shotton (2018). Open Citation: Definition. Figshare.

[3] David Shotton, Marilena Daquino and Silvio Peroni (2020). In-Text Reference Pointer Identifier: Definition. Figshare.

[4] Marilena Daquino, Silvio Peroni and David Shotton (2019). The OpenCitations Data Model. Version 2.0. Figshare.

Cite this article as: davidshotton, "Introducing InTRePIDs – In-Text Reference Pointer Identifiers," in OpenCitations blog, 30/01/2020,

Open Citations and Semantic Publishing

Given the renewed interest among publishers in the Open Citations Corpus, following the decisions by Nature Publishing Group, publisher of Nature, and by the American Association for the Advancement of Science, publisher of Science, to open their citation data for inclusion in the corpus, I thought it would be helpful to provide links to videos of two conference presentations I gave that describe the Open Citations Corpus in the context of our other work in the area of semantic publishing.

The first of these was an invited contribution with the title Enriching Scientific Citations to Facilitate Knowledge Discovery, given to publishers at the Scientific, Technical and Medical Publishers Innovations Seminar 2010 entitled “Flows in Flux: how publishing technologies change the researcher’s life”, held in London, UK, on 3rd December 2010:




The second presentation, given almost a year later, was an invited contribution with the title The SPAR (Semantic Publishing and Referencing) Ontologies and the Open Citation Corpus, given to librarians at SWIB11 (Semantic Web in Bibliotheken; Semantic Web in Libraries Conference 2011) “Scholarly Communication in the Web of Data”, held in Hambrug, Germany on 30th November 2011:




Further details of the Open Citations Corpus and the SPAR ontologies, and their applications are, of course, given in this Open Citations and Semantic Publishing blog, for which the following two posts provide the best introduction for those coming here for the first time:

JISC Open Citations Project – Final Project Blog Post

Introducing the Semantic Publishing and Referencing (SPAR) Ontologies

Cite this article as: davidshotton, "Open Citations and Semantic Publishing," in OpenCitations blog, 18/06/2012,

Access to Citation Data

The JISC, in response to its invitation to tender, has recently funded Curtis+Cartwright Consulting Ltd, a research and strategy consultancy, to undertake an independent study entitled Access to Citation Data: A Cost-Benefit and Risk Review and Forward Look.  Evidence gathering for the study has just started, and the consultants are due to produce a report on this subject by next February.  This report will be one of a series of high profile reports that the JISC is commissioning to inform the UK FE/HE sector on key issues relating to digital infrastructure.

The JISC has invited me to serve on the steering group for this study, which will in particular be reviewing the Open Citations Corpus, both to provide intelligence and to act as a case study and as a potential recommended corpus of citation data.

I look forward to working with the review team, starting later this month, and anticipate that we will benefit from this investigation as well as contribute to it, since we anticipate that Curtis+Cartwright will be able to advise on sustainable business models for such open access citation data corpora.

Cite this article as: davidshotton, "Access to Citation Data," in OpenCitations blog, 12/06/2012,

Why publishers should open their references

Why should the publishers of subscription-access journals, who presently generate income from the sale of access to peer-reviewed full text scholarly articles, be willingly open the reference lists of these articles, and contribute these to the Open Citations Corpus for publication as open linked data? I would like to suggest the following reasons:

1. There is a general move towards open data, which is widely regarded as a common good. This includes citation data, i.e. bibliographic references from one article to another (in RDF Turtle format: A cito:cites B . ).
2. The reference lists at the end of journal articles are works of scholarship by the authors, who have chosen to include certain references and exclude other potentially citable papers from the reference list. However, the references themselves are simply items of bibliographic data, formatted according to the journal style, and do not benefit from the author’s creative input.
3. The reference list, together with the front matter (including the bibliographic information about the article itself) and the abstract, has traditionally been included within the copyright protection enjoyed by the article as a whole. However, the bibliographic information about the article and the article’s abstract are commonly made freely available, for example through PubMed. This same openness should now be afforded to the reference list within each article.
4. There is a home for such reference citation data: the Open Citations Corpus has been specifically created to house and publish scholarly bibliographic citation data, and is now preparing to welcome article reference lists from subscription-access journals, to supplement those already contributed from open-access journals.
5. For those publishers who already contribute their reference information to CrossRef as part of its Cited-By Linking service, this can be accomplished without any change to the publisher’s own publishing workflows, just by giving permission for CrossRef to flag the articles of certain journals as having open references. Open Citations intends to collaborate with CrossRef by harvesting the reference lists from such flagged articles, parsing them into RDF, and adding them to the Open Citations Corpus. Provided that the references are already being submitted to CrossRef, no work will have to be done by the publisher, and no changes in publishing procedure will be involved.
6. Open Citations will publish each reference list as an independent RDF Named Graph, with a unique URI, thereby protecting the integrity of the article reference list as a unit of scholarship, the source of which will be explicitly acknowledged.
7. The open citations data will then be offered back to publishers to use as they wish, e.g. for visualization of citation networks, calculation of metrics, etc., providing easier and more usable access to their own citation data than is currently afforded by commercial providers, who do not provide such data in linked data format.
8. Publishers will also be free to host their own open citations data, should they wish to do so.
9. For the majority of publishers, who would still receive subscriptions on the full articles themselves, opening their article reference lists in this way will cost nothing in terms of lost revenue.
10. Indeed, participation in the Open Citation Corpus will bring the following benefits to subscription-access publishers:
– Access to services to be built over the aggregated open citations data, for example an automated reference correction service available to editors upon receipt of a manuscript, for the automated pre-publication correction of errors in reference lists prior to article publication.
– Increased exposure to users of the references to the publisher’s own journal articles – a form of advertising. While at first coverage among subscription-access publishers will be incomplete, this expanding Open Citation Corpus will, in true Web 2.0 style, become more useful the more publishers participate.
– Even while coverage is incomplete, the Open Citations Corpus by its very nature contains reference citations to all the key papers published in every field covered – currently to all the key papers published in every biomedical field, enabling readers more easily to identify and find the most highly cited papers of each contributing publisher.
– Opening citations data will result in white-listing and general good-will from funding agencies, government and other advocates of open data, who might otherwise mandate publication by grantees in alternative open-access journals.
– Opening citations data will lead to support from scholars and researchers themselves, who wil be more inclined to publish in that publisher’s journals, feeling that at last the publishers would be giving back to them some of their own data, rather than selling it back to them as at present.

As my next blog post shows, one leading subscription-access publisher is now willing to open its journal article references in the way I have suggested.  Others who would like to so the same should contact me at <>.

Five Stars Ontology

To accompany today’s publication in D-Lib Magazine of the article The Five Stars of Online Journal Articles – a framework for article evaluation highlighted in the previous post, I have today also published The Five Stars Ontology, a simple ontology written in OWL 2 DL that forms part of SPAR, a suite of Semantic Publishing and Referencing Ontologies. It is intended for use by publishers and others wishing to encode Five Stars ratings, such as those exemplified in the D-Lib article, in machine-readable form, so they can accompany other machine-readable metadata for the article.  To exemplify this, the following RDF graph, shown in turtle notation, gives the Five Stars ratings for the D-Lib article itself:

     fivestars:hasPeerReviewRating “3”^^xsd:nonNegativeInteger ;
     fivestars:peerReviewRatingComment “Post-publication responsive
          peer review of the preprint.” ;
     fivestars:hasOpenAccessRating “4”^^xsd:nonNegativeInteger ;
     fivestars:openAccessRatingComment “Gold/libre open access
          without author fee!” ;
     fivestars:hasEnhancedContentRating “1”^^xsd:nonNegativeInteger ;
     fivestars:enhancedContentRatingComment “Plentiful Web links in
          text and to all references. No additional semantic
          enhancement of text.” ;
     fivestars:hasAvailableDatasetsRating “0”^^xsd:nonNegativeInteger ;
     fivestars:availableDatasetsRatingComment “Not applicable.” ;
     fivestars:hasMachine-readableMetadataRating “1”^^xsd:nonNegativeInteger ;
     fivestars:machine-readableMetadataRatingComment “Structural
          markup in HTML only.” ;
     fivestars:hasOverallFiveStarsRating “9”^^xsd:nonNegativeInteger ;
     fivestars:overallFiveStarsRatingComment “The nature of this
          article, being a position paper rather than a research
          paper with primary research data, has influenced the
          overall rating obtained.” .
Cite this article as: davidshotton, "Five Stars Ontology," in OpenCitations blog, 16/01/2012,
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