This blog post is the first of a series which will highlight some of the ways OpenCitations is currently adopted and used by the community. This series also gives us the chance to thank our users for trusting OpenCitations and for giving us the opportunity to improve our services through their feedback. With you, OpenCitations is a drop generating countless circular waves of innovation in research.
The dblp computer science bibliography (https://dblp.org/) is the on-line reference for bibliographic information on major computer science publications, with the mission “to support computer science researchers in their daily efforts by providing free access to high-quality bibliographic meta-data and links to the electronic editions of publications”.
Currently, dblp indexes over 7 million publications, published by more than 3.4 million authors. Despite the small team of editors from the Schloss Dagstuhl, which run the service, in the last few years, dblp has grown from a small-scale server to a service used by thousands of people worldwide.
In 2022, they released the dblp Knowledge Graph, and in June 2024 they released a first major extension to the dblp KG by adding publications venues (e.g., journals and conference series) as first-class entities to the graph. This new release also includes a new property (i.e. dblp:omid) to link dblp publications to their OpenCitations Meta Identifier (OMID). This ID is crucial for making use of open citation and reference data available in OpenCitations since it is the internal identifier we use to identify every bibliographic entity (publications, authors, venues, etc.) stored in our system.
Indeed, the dblp computer science bibliography doesn’t collect and store citation information itself, but instead ingest directly the open citation data released by OpenCitations. Using the linkage provided by OMID, the dblp users can perform citation analyses using its SPARQL query service.
Dblp has published a detailed guide to help “beginners” find data in the dblp Knowledge Graph, and you can find it here.
If you are looking for more information about the dblp project, please explore their exhaustive F.A.Qs.
The first month of the new year has almost come to an end, and we at OpenCitations have dedicated these weeks after the holiday season to retrace the progress we reached as an open infrastructure throughout 2023, an activity that has become a tradition in the past few years. Looking back to the achievements has proven to be a great practice for our team not only to honour and celebrate the success, but also to visualize the points to focus on to best serve our community when planning future activities. During 2023 we have reached numerous milestones, which can be summarized under the two main topics: technical developments and new connections.
Technical Developments
1. New sources
At the end of 2022, we had enriched our collection with two more sources, PubMed and DataCite, which were added to Crossref. During 2023 we completed the integration between OpenCitations and the OpenAIRE Graph as concerns open bibliographic and citation data, and in December we included the DOI-to-DOI citation links derived from the Japan Link Center (JaLC). As a result, by the end of 2023, OpenCitations counted almost 2 BILLION open citations available in its collection.
2. A brand-new workflow for an easier consultation
The progressive integration of new sources is one of OpenCitations’ main objectives. However, the recent source increase made it necessary to design a new ingestion workflow to facilitate their management. In November 2023 OpenCitations launched the new workflow, designed to produce just two comprehensive collections: OpenCitations Index, collecting open citation data, and OpenCitations Meta, containing open bibliographic metadata of citing and cited entities involved in citations. This evolution is making it simpler for OpenCitations users to consult the data maintained by our infrastructure without reducing the information content and accuracy.
New connections
3. A great event for the community
In October 2023, OpenCitations hosted the Workshop on Open Citations and Open Scholarly Metadata 2023 (WOOC 2023) in the historical location of the University of Bologna. After two online editions, it was exciting to meet in person the researchers, policy makers and service providers who in the past few years have collaborated with OpenCitations in many ways. For two days, almost 80 people from all around the world gathered to discuss the theme “The value of open scholarly metadata for Research Assessment purposes”. The event gave us a picture of a young and enthusiastic research community, aiming to collaborate among different countries to spread the message “Open Science is Better Science”. If you couldn’t attend the workshop, but are interested in the theme, you can find all the presentations from WOOC 2023 on Zenodo. Stay tuned on the WOOC X account and website, we will announce the 2025 edition later in 2024!
4. New and old partnerships
OpenCitations’ main activities, besides the technical development, are tied to its participation in R&D and funded projects. Since 2019, we have started building a network of collaborations all around the world with institutions which share the same Open Science values. In 2023, our involvement in the SCOSS 2nd funding cycle has come to an end. In the last four years SCOSS has worked as an international showcase for OpenCitations, making it possible for us to get known by many institutions on an international level. We are now proud to keep on collaborating with SCOSS within the SCOSS Family network, a community of practice gathering all the OS infrastructure involved in the SCOSS funding cycles. During 2023, the SCOSS Family was involved in four internal sprints dedicated to planning four actions – collectively advocating the importance of open infrastructure, communications kit for ‘the post-SCOSS phase’, collective self-assessment towards POSI, and map open science infrastructure ecosystem including SCOSS infrastructures – and the initial outcomes were presented in the Scoss Family Public Webinar 2023. The SCOSS family will continue its activities throughout 2024.
In 2023, we also celebrated the update of the Principles of Open Scholarly Infrastructure (POSI), as a result of the work conducted since 2022 with the “POSI-Adopters” group to make the principles clearer and more inclusive. In June, the OpenAIRE-Nexus project, started back in 2021, formally came to an end, with the release of the index of citations from OpenAIRE. OpenCitations is part of the OpenAIRE Catalogue, which also links to the exhaustive and engaging materials we produced within the project. However, we continue to meet the familiar and friendly faces from the OpenAIRE team in the meetings of GraspOS, a project which started in January 2023 involving several European Open Science institutions, services and infrastructures. In GraspOS, OpenCitations is involved as one of the Data Asset Sources, which provides both the raw and the structured data that can be used to directly support OS-aware RRA processes or to calculate indicators or collect evidence that can be of value in these processes. The researchers from OpenCitations and the University of Bologna are contributing to GraspOS by developing a tool for the extraction of citation data from PDFs, enriched with additional information on the reasons for citing (because the citing entity reuses a method described in the cited entity, for agreeing with the claims of the cited entity, etc.) through the implementation of machine-learning systems. Also, together with GraspOS, OpenAIRE, and many other Coalition members, the OpenCitations personnel at the University of Bologna is among the participants of the CoARA Working Group Towards Open Infrastructures for Responsible Research Assessment (OI4RRA), to work on enabling the research community to be in full control of the data and indicators it relies on in assessments. OpenCitations is always seeking interest from new institutions, and we are proud that our visibility is boasted by some reliable catalogues including the OpenAIRE catalogue, the EOSC Marketplace and, from 2023, also the JISC Catalogue. And soon you’ll find us in a new tool – stay tuned to find out!
5. Our way towards sustainability
OpenCitations is increasingly gaining a prominent role in the community, establishing itself as a key player in changing research evaluation practices towards greater openness and equity. In recent years, OpenCitations has grown a lot both technically and in popularity, and for this we want to thank the number of members and donors who, even in the last year, have decided to support us and believe in our infrastructure. During 2023, our two Internal Working Groups “Governance Evolution” and “Community Building”, thanks to the participation of some members of the International Advisory Board for OpenCitations, reflected on the sustainability of OpenCitations, discussing strategies both at the management and communication level to ensure the integration of new actors into its subsistence. The groups will continue to meet throughout 2024, and some of the results will be released in the coming months. However, there is still a long way to go towards sustainability, and OpenCitations’ existence still relies on community funding, which is why OpenCitations still needs support: if you are looking for more information on why and how to support OpenCitations, you can read our recent call for action here. Thank you!
6. Enlarging our team
It is thanks to the support received that OpenCitations was able to develop its yearly activities, and we are happy to announce that in 2023 a new software developer, Elia Rizzetto, and a new PhD student, Francesca Cappelli, have joined our team to make this progress possible. The OpenCitations team now has a total of 12 members, including research fellows, PhD students, MA students, a Reseach Manager, an Assistant Professor and OpenCitations’ Directors. The number is expected to grow soon, as OpenCitations has recently released a call for the hiring of a System Administrator for technical infrastructure management.
As can be seen from this brief (and detailed) report of some of the developments that have marked our 2023, OpenCitations is increasingly emerging as a community-based and community-guided infrastructure organization, in which there is an indissoluble link between technical progress and the network of people who guide the planning, work on implementation, and use or promote our services. Throughout 2024, OpenCitations aims to focus even more on harmonizing the relationship between the technical infrastructure and the community, developing more intuitive ways of accessing services and communications, and continuing to weave relationships with new research groups. We look forward to seeing what the year ahead will bring with it, and we hope you will continue to be with us.
Blog post by Ivan Heibi (University of Bologna), Arianna Moretti (University of Bologna) and Chiara Di Giambattista (University of Bologna).
In the past five years, the OpenCitations data has been enriched with numerous new indexes of open citation data from different sources. However, the quantity and diversification of the ingested information have raised several issues, which recently made it essential to conduct a complete revision of the ingestion workflow. The result was a revolution in the way OpenCitations data is delivered. In this blog post, we will explain the context and challenges raised by the old procedure. Then, we will present the new ingestion workflow, designed to produce just two comprehensive collections: OpenCitations Index, collecting open citation data, and OpenCitations Meta, for the open bibliographical metadata.
Once upon a time, there were five OpenCitations indexes…
In 2018, OpenCitations released the kickoff version of its first citation index, COCI (citations from Crossref), which contained around 300 million citation links derived from the subset of the reference lists in the Crossref database, where citing and cited entities were identified using Digital Object Identifiers (DOIs). COCI gathered citations with associated metadata in compliance with the recommendations from the Initiative for Open Citations (I4OC) that citation data should be structured, separable, and open, thus marking a turning point by providing a disruptive and free and open alternative to earlier sources such as Google Scholar, which provided freely accessible data although not downloadable, and Web of Science or Scopus, which demanded paid access.
In a short time, COCI became a competitive and trusted index of citation data, used by numerous institutional repositories, including B!son and Optimeta. In 2021, COCI was taken into account in a comparative study with the most relevant sources in the landscape, including the proprietary ones, which showed its coverage approaching parity with those of the other sources involved in the analysis (Microsoft Academic, Scopus, Dimensions, and Web of Science). At the time of its most recent update in January 2023, COCI counted more than 1.4 billion citations. The reason behind this outstanding number lies in several factors, including Elsevier’s endorsement of the Declaration on Research Assessment (DORA) in December 2020, leading to the open release via Crossref of the reference lists of the articles published in all its journals, and confirming the value of initiatives such as the Initiative for Open Citations (I4OC).
However, before this change of heart, in 2019 OpenCitations had tried to narrow the open citations coverage gap by launching its second index, the Crowdsourced Open Citations Index (CROCI). This index allowed publishers and scholars to contribute directly by uploading crowdsourced open citations into the OpenCitations infrastructure.
In December 2022, a new concrete step towards a factual plurality of OpenCitations indexes was taken by the ingestion of new data sources into the infrastructure, with the publication of the inaugural dumps of DOCI (citations from DataCite) and POCI (citations from PubMed). In June 2023, the first version of the OROCI (citations from OpenAIRE) dump was released too, and JOCI (citations from JALC) is expected to be available by the end of November 2023, for a total of five collections from different sources.
Why a new workflow? The issues with multiple sources management and new challenges
While having such a variety and richness of indexes helped present the extent of OpenCitations sources, the recent increment in the number of sources and the diversification of data integrated led to two primary issues:
the necessity to handle the ingestion of new identifier types in a DOI-based software infrastructure, and
the consequent possibility of encountering the same citation expressed by several sources with different identifiers.
Moreover, it soon became evident the need to optimize the reuse of the already developed software components to facilitate the metadata crosswalk processes between the new sources’ data models and the OpenCitations Data Model, with the aim to define a functional and easily extendable workflow to be easily reused when it comes to incorporating new data sources, which should be:
sufficiently generic to establish a globally unique procedure;
customizable enough to capture the necessary information within each of the specific data models and formats.
As a solution, we decided to use OpenCitations Meta, the new OpenCitations database and tool for managing bibliographic data related to the publications involved in the citations. OpenCitations Meta makes it possible to assign each entity involved in a citation an internal identifier, nominally the OpenCitations Meta Identifier (OMID), to which all the associated persistent identifiers of the same publication are redirected.
As a result, the allocation of an OMID for each bibliographic resource also enabled the unambiguous identification of each citation, regardless of the persistent identifier schema originally used by the data source to identify the resources. This approach allowed us to perform data deduplication and finally make all the sources’ contributions converge into a unified index containing all the unique citations managed by OpenCitations, expressed as OMID to OMID citation links.
The revised workflow
The new workflow is based on three main components with the benefit of optimizing the process both in terms of computational cost and in terms of flexibility. As shown in Fig. 1, in a preliminary step, source-specific software converts the input dataset – structured according to the source data model – to extract two OpenCitations Data Model compliant data collections in tabular format for bibliographic metadata and citation data, respectively.
The following steps are common to the process of each dataset.
STEP 1: The bibliographic metadata collection is used as input for the META software. At this stage, it is checked whether or not the bibliographic entities have been previously integrated into our infrastructure (coming from other data sources). If so, the existing OMID is linked also to the new alternative identifiers of the new bibliographic resources. New metadata values, if any, are also integrated. A new OMID identifier is produced for entities never previously encountered, uniquely representing the bibliographic resource in OpenCitations. The outputs of the process are: (I) an updated version of the OpenCitations Meta collection that also includes the metadata of the bibliographic entities provided by the new source, and (II) a collection of provenance data. An internal database is constantly refreshed to preserve correspondence between IDs and the associated internal OMIDs.
STEP 2: Starting from the collection of citations expressed as directional links between identifiers of potentially any type (e.g., DOI-DOI, PMID-PMID, PMC-PMID, etc.), the INDEX software queries the internal database mapping IDs to OMIDs to produce an updated version of the OpenCitations Index: unique citations expressed as OMID-OMID links in different formats, accompanied by their corresponding provenance data.
Fig. 1: An overview of the data ingestion workflow, starting from the data source-specific conversion and production of citations and bibliographic metadata tables, progressing through the META process and the assignation of an OMID identifier to each bibliographic record involved in a citation, and culminating with the exposition of the OpenCitations Index collection of OMID-OMID unique citations.
What we have now: The OpenCitations Index
From now on, OpenCitations will no longer display an index of citation data for each source. Instead, we will publish a single collection of citations into which the contributions from each of the sources will flow, which we will simply call ‘The OpenCitations Index‘. The first version of this unified index of OMID-OMID citations is posted on Figshare. It was produced in RDF, CSV, and SCHOLIX formats, together with a collection of its provenance information, provided in RDF and CSV formats. For each citation, it is possible to trace the source of the information by consulting the Provenance data collection, thanks to the http://www.w3.org/ns/prov#atLocation property, which defines the location of each citation.
This new solution has the benefit of simplifying the consultation of the data maintained by our infrastructure without reducing the information content. In addition, by including efficient handling of the deduplication problem, the new Index not only provides accurate data on the exact number of unique citations exposed by the framework but also verifies the individual contribution of each source, as well as their overlapping data (Fig. 2).
Fig. 2: An overview of the number of citations stored in the OpenCitations Index as of October 31, 2023. The diagonal cells in the table (highlighted in yellow) show the unique contribution of each collection to the OpenCitations Index, while the other cells represent the citations that are shared between the collections. More in detail, the green cells show the overall input of each source, while the pink cells represent the number of overlapping citations between two data sources.
Currently, the Index contains almost 2 billion unique citations. By the end of November, a new version of the collection will be published, including the contribution of the new Japan Link Centre (JaLC) source.
How to access the OpenCitations Index data
To maximize the reuse of the exposed information and to ensure the greatest possible interoperability, the collection will always be published on Figshare in all formats listed above. In addition, the data will be accessible via an API, a SPARQL endpoint, and a web interface.
The redesign of the ingestion workflow marks a fundamental step for OpenCitations towards a more intuitive and simple access to our services while always preserving and improving the quality of our data. If you need further information on how the new workflow works, please visit our website, contact us at contact@opencitations.net or leave feedback and/or suggestions in the dedicated card on our public roadmap to help us improve our services and communications. Thank you!
In OpenCitations, we like to define our infrastructure organization as “community-based” and “community-driven”, and we really mean it. The support coming from the number of academic libraries and consortia coming after OpenCitations’ involvement in the 2nd SCOSS funding cycle has made it possible, starting from 2020, to make OpenCitations develop from a small university project based on time-limited grant incomes to being an open infrastructure globally recognized for the provision of open citation data and bibliographical metadata. We want to thank all our members and donors, for trusting our mission and sustaining OpenCitations activities with their continuous and generous support, despite the pandemic and post-pandemic times.
While retracing our work in the last three years, we are astonished by the achievements our team has accomplished, and by how in such a limited time frame OpenCitations has approached David Shotton’s initial vision (who is one of the co-directors of OpenCitations), when he first shaped the project back in 2010. Here are just some of the technical developments that have marked the last years:
in 2022, we released the two new OpenCitations indexes of open citations, DOCI (citations from DataCite) and POCI (citations from PubMed);
we expanded our collection besides the citation data by releasing OpenCitations Meta, a database storing and delivering bibliographic metadata for all the publications in the OpenCitations Indexes, including the publication’s title, type, venue (e.g. journal name), volume number, issue number, page numbers, publication date, identifiers and details of the main actors involved in the document’s publication (the names of the authors, editors, and publishers);
as of October 2023, OpenCitations Indexes contain information on 1.82 billionunique open citations.
However, the most significant achievements for OpenCiations in the last years have been the creation of a prolific network of collaborations with other Open Science projects, such as OpenAIRE-Nexus, RISIS2 and GraspOS, and the establishment of a structured team, involving young researchers and PhD students, whose work at the University of Bologna has made it possible to work on the technical developments day by day.
Our results are a strong indicator of the growing sensibility on the theme of the open provision of bibliographical metadata and citation data, of which Open Citations is at the forefront as a founding member of the Initiative for Open Citations (I4OC) and the Initiative for Open Abstracts (I4OA). The effort and awareness campaigns led by these initiatives, by DORA and the Open Science community as a whole, have led more and more publishers to a change of heart and to open their reference lists. OpenCitations is an integral part of an ongoing process of transformation of the research environment, and we have collected and interpreted some of the needs of the academic community to plan our future activities and developments. We still need your help and support to make it possible to maintain and improve our infrastructure and to sustain the team working at OpenCitations.
If you believe in
the importance of open bibliographic data for the creation of reproducible metrics for research assessment exercises
the power of the scholarly community to change existing practice by reclaiming ownership of its own data– and you want to become an active part of this change
please consider supporting OpenCitations either via membership or donation. You can find all the information on membership on our website at https://opencitations.net/membership, or you can ask for information by contacting us at membership@opencitations.net.
Together, we can work to create an open and inclusive future for science and research.
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 RISIS2we 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: https://opencitations.net/membership.
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.
We’re happy to announcePOCI, the OpenCitations Index of PubMed open PMID-to-PMID citations, 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). 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.
Each citation (i.e. an individual of the class cito:Citation) is identified by an URL structured as follows:
https://w3id.org/oc/index/poci/ci/[[OCI]].
Open Citation Identifiers
Each Open Citation Identifier [[OCI]] has a simple structure: the lower-case letters “oci” followed by a colon, followed by two numbers separated by a dash (e.g. https://w3id.org/oc/index/poci/ci/01600102060800080706-016002060909030401), in which the first number identifies the citing work and the second number identifies the cited work.
For citations in which the citing and cited works are identified by PMIDs, which includes all the POCI citations, the OCI is created in the following manner, as explained more fully here. Each converted numeral part of OCI is prefixed by a 0160, which indicates that NIH is the supplier of the original metadata of the citation (as indicated at http://opencitations.net/oci).
are available as dumps on Figshare in CSV, N-Triples and Scholix.
What is an Open Citation Index?
A citation index is a bibliographic index recording citations between publications, allowing the user to establish which later documents cite earlier documents. The current indexes available in OpenCitations are:
This blog post is the first of a series dedicated to the description and promotion of OpenCitations Meta.
In addition to OpenCitations’ Citation Indexes, OpenCitations is pleased to announce a new service: OpenCitations Meta, a database which stores and delivers bibliographic metadata for all publications involved in the OpenCitations citation 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 publication of the document, i.e., the names of the authors, editor, and publisher, each with its own additional metadata and identifier (e.g. ORCID).
Why OpenCitations Meta?
OpenCitations Meta represents a major infrastructural advance for OpenCitations, since it addresses and overcomes some of the limitations previously experienced, providing three major benefits:
1. It permits OpenCitations to respond more quickly to user requests for information.
The OpenCitations Citation Indexes contain citation-related metadata, such as the identifiers for the citing and cited publications, the citation timespan, and other citation-specific metadata, for example whether or not the citation is a self-citation. However, metadata for the citing and cited publications were hitherto not stored by OpenCitations, but rather obtained by on-the-fly API calls to external services such as Crossref, Datacite and ORCID. This dependence on responses from external services significantly slowed the delivery of the required information to users. Since OpenCitations Meta stores in-house the same bibliographic metadata for the citing and cited publications as was previously obtained by those external API calls, the former temporal limitation is now alleviated, thereby accelerating response times for requests made to the OpenCitations REST API services, and thus better supporting our users.
2. It permits OpenCitations to index citations involving publications that lack DOIs.
The second problem that OpenCitations Meta addresses goes beyond the OpenCitations infrastructure and addresses a more general issue of digital publishing, particularly in the Humanities, namely that many scholarly publications lack persistent identifiers such as DOIs and PMIDs. The citations indexed by OpenCitations are themselves treated as first-class data entities, each bearing a unique persistent Open Citation Identifier. According to the Open Citation Identifier definition [1], the entities linked by an indexed citation need to be identified by a persistent identifier (PID) from a recognised scheme such as the Digital Object Identifier scheme. For example, COCI [2] records the DOIs of the citing and cited entities. However, the association of DOIs with published documents is not uniformly spread across all scholarly fields. Indeed, in Scopus and the Web of Science Core Collection, only 50% of the current publications in the Art and Humanities are associated with a DOI [3]. In addition, in many fields, particularly in the Humanities, references are required to pre-digital historical primary sources that lack PIDs. OpenCitations Meta solves this problem by assigning a unique OpenCitations Meta Identifier (OMID) to each document. In this way, it can record the metadata of publications lacking an external identifier and store the citations involving them in one of the OpenCitations Indexes.
3. Use of the OpenCitations Meta Identifier permits de-duplication of records of bibliographic resources having more than one external identifier, such as a DOI and a PMID.
While many scholarly publications lack external PIDs, many others will have more than one external PID, for example a DOI for the record of that publication in Crossref, and a PMID for the record of that same publication in PubMed. This is perfectly legitimate. Since OpenCitations Meta assigns a unique OpenCitations Meta Identifier (OMID) to every entry, and in addition records all external identifiers already assigned to that publication, for example its DOI and its PMID, this permits the automatic de-duplication of such publications within OpenCitations Meta.
The data model
OpenCitations uses Semantic Web technologies to encode all its data, including the bibliographic metadata now stored in OpenCitations Meta. For this purpose, the OpenCitations Data Model (OCDM) [4] uses classes and properties defined in the SPAR Ontologies to represent:
bibliographic entities (fabio:Expression), e.g. a journal article
identifiers (datacite:Identifier), e.g. a DOI
agent roles (pro:RoleInTime), e.g. an author
responsible agents (foaf:Agent), e.g. a person
publication format details (fabio:Manifestation), e.g. first and last pages
Figure 1 depicts relationships between some of the various entities.
Figure 1. A small subset of the OCDM used in OC Meta. Yellow rectangles represent classes, green polygons represent data types, while blue and green arrows represent object properties and data properties, respectively
In addition to storing data, great importance is given to recording provenance and change-tracking of OpenCitations Meta entries, including the agent who processed a specific entity, i.e. creating, deleting, modifying or merging it, the date and time on which this action was performed, and the primary source of the data.
The data
At the release date, 20th December 2022, OpenCitations Meta is using Crossref as its primary source, with the purpose of hosting the metadata describing all citing and cited publications involved in the 1,392,036,835 citations within COCI [2]. From a quantitative point of view, OpenCitations Meta presently contains metadata describing 87,321,593 bibliographic entities, 277,750,235 authors, 2,359,301 editors, 17,268 publishers, and 710,226 publication venues (e.g. journal names). Editors and authors have been counted by their roles, without at this stage disambiguating individuals with the same name. Conversely, bibliographic entities, publishers, and publication venues were counted by their OMIDs. In total, the OpenCitations Meta triplestore presently contains 3,298,335,581 triples.
In the following days, OpenCitations Meta will be expanded to include bibliographic metadata for all citing and cited publications involved in the 169,822,752 citations from DataCite within DOCI, released on 13th December 2022.
Table 1 lists the various types of bibliographic resource represented in OpenCitations Meta. The current dataset contains mostly journal articles (60,683,841), which exceed the number of book chapters (6,288,351) by about ten times and the number of conference proceedings articles (4,917,568) by about twelve times.
OpenCitations Meta allows the users to explore the contained metadata either via SPARQL (https://opencitations.net/meta/sparql) or via the OpenCitations Meta REST API (https://opencitations.net/meta/api/v1). In particular, the API will retrieve a list of bibliographic resources and related metadata starting from one or more identifiers including an OpenCitations Meta Identifier (OMID), an author’s ORCID, or an editor’s ORCID.
Finally, all the metadata and provenance information present in OpenCitations Meta are available as dumps in RDF (JSON-LD) or CSV format under a CC0 licence. Every time a citation index is updated, we will update OpenCitations Meta.
Conclusion
OpenCitations Meta makes search operations on the metadata of scholarly publications involved in the citations indexed in the OpenCitations Citation Indexes faster, since the metadata are now held in-house. The use of OpenCitations Meta Identifiers (OMIDs) permits OpenCitations Meta to act as a mapping hub to disambiguate and unify information on single publications identified by more than one external identifier, and makes it possible to characterise citations involving resources lacking an external identifier.
In the future, we will introduce a new API operation to perform text searches. Moreover, OpenCitations Meta will incorporate metadata from other sources in addition to Crossref. As mentioned above, work is already under way to ingest bibliographic metadata from Datacite for the publications involved in the citations newly indexed in DOCI. This will soon be followed with similar OpenCitations Meta entries relating to a new citation index, POCI, containing citations from PubMed, to be released later this year. PubMed metadata will be made available by February 2023.
Heibi, I., Peroni, S. & Shotton, D. Software review: COCI, the OpenCitations Index of Crossref open DOI-to-DOI citations. Scientometrics 121, 1213–1228 (2019). https://doi.org/10.1007/s11192-019-03217-6
Gorraiz, J., Melero-Fuentes, D., Gumpenberger, C., & Valderrama-Zurián, J.-C. (2016). Availability of digital object identifiers (DOIs) in Web of Science and Scopus. Journal of Informetrics, 10 (1), 98–10. https://doi.org/10.1016/j.joi.2015.11.008
We’re excited to introduce DOCI, the OpenCitations Index of Datacite open DOI-to-DOI citations, a new tool containing citations derived from publications bearing DataCite DOIs to other DOI-identified publications, harvested from DataCite. 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.
For citations in which the citing and cited works are identified by DOIs, which includes all the DOCI citations, the OCI is created in the following manner, as explained more fully here. Each case-insensitive DOI is first normalized to lower case letters. Then, after omitting the initial doi:10. prefix, the alphanumeric string of the DOI is converted reversibly to a pure numerical string using the simple two-numeral lookup table for numerals, lower case letters and other characters presented at https://github.com/opencitations/oci/blob/master/lookup.csv. Finally, each converted numeral is prefixes by a 080, which indicates that DataCite is the supplier of the original metadata of the citation (as indicated at http://opencitations.net/oci).
A citation index is a bibliographic index recording citations between publications, allowing the user to establish which later documents cite earlier documents. The current indexes available in OpenCitations are:
Fig. 1: An overview of the data ingestion workflow, starting from the data source-specific conversion and production of citations and bibliographic metadata tables, progressing through the META process and the assignation of an OMID identifier to each bibliographic record involved in a citation, and culminating with the exposition of the OpenCitations Index collection of OMID-OMID unique citations.
