OpenCitations described

OpenCitations is an infrastructure organization for open scholarship dedicated to the publication of open bibliographic and citation data. We at OpenCitations are proud to announce the publication, in the first issue of Quantitative Science Studies, of a canonical paper in which we introduce and describe OpenCitations and outline its achievements and goals [1].

Here, I outline the contents of our paper, and provide definitive links on the topics described. Many of these topics have been the subjects of earlier blog posts.

This paper appears in the first Special Issue of QSS, dedicated to the description of the bibliometric data sources that lie at the heart of scientometric research, which aims to characterize the most important data sources currently available and to show how they differ in various dimensions, for instance in the data they provide, their level of openness, and their support for making research reproducible. The first three papers in this special issue cover the most important commercial bibliographic data sources: Web of Science (Clarivate Analytics), Scopus (Elsevier), and Dimensions (Digital Science), while the remaining three articles describe open data sources: Microsoft Academic, Crossref and OpenCitations.

In the introduction to our own paper, we describe the origins of OpenCitations, discuss the growth and benefits of open science, and introduce the Semantic Web techniques used at OpenCitations for recording and publishing our data. We then go on to describe OpenCitations’ services and data, namely Open Citation Identifiers, the OpenCitations Data Model, the SPAR (Semantic Publishing and Referencing) Ontologies, the OpenCitations Corpus, and the OpenCitations Indexes of citation data, of which the first and largest is COCI, the OpenCitations Index of Crossref open DOI-to-DOI citations, that currently holds information on over 624 million citations. We conclude our survey of OpenCitations’ services and data by outlining the generic open source software developed at OpenCitations, including OSCAR, the OpenCitations RDF Search Application for searching over RDF datasets, LUCINDA, OSCAR’s associated OpenCitations RDF Resource Browser, and RAMOSE, OpenCitations’ application for creating REST APIs over SPARQL endpoints, thus opening Semantic Web datasets to those not familiar with SPARQL, the RDF query language.

In the second half of the paper, we describe OpenCitations as an organization in terms of its compliance with the principles for the sustainability of open infrastructures proposed by Bilder, Lin and Neylon (2015) [2], and report the selection of OpenCitations by the Global Sustainability Coalition for Open Science Services (SCOSS) as an open infrastructure organization worthy of crowd-funding support by the stakeholder community. We then provide usage statistics for our datasets and web site, and describe the adoption of OpenCitations data and services by the community, before concluding with a forward look at our proposed developments of OpenCitations activities.


[1] Silvio Peroni and David Shotton (2020). OpenCitations, an infrastructure organization for open scholarship. Quantitative Science Studies 1 (1): 428-444.

[2] Geoffrey Bilder, Jennifer Lin and Cameron Neylon (2015). Principles for open scholarly infrastructures. Figshare.

Cite this article as: davidshotton, "OpenCitations described," in OpenCitations blog, 26/02/2020,

Open Citations is dead. Long live OpenCitations.

OpenCitations logo 50% with words greyBG

In October 2015, I asked Silvio Peroni, my long-term colleague in the development of the SPAR Ontologies, to become Co-Director of the Open Citations Project, and to work with me in taking forward the prototype Open Citations Corpus (OCC), originally developed at the University of Oxford with the support of Jisc, with the aim of developing it into a production service of real use to scholars.

The result is OpenCitations, a new instantiation of the OCC hosted by the Department of Computer Science and Engineering of the University of Bologna, based on a new metadata schema and employing several new technologies to automate the ingestion of fresh citation metadata from authoritative sources.

Since the beginning of July 2016, OpenCitations has been ingesting and processing accurate bibliographic references harvested from the reference lists of scholarly papers available in Europe PubMed Central, enriched by metadata from Crossref. These scholarly citation data are described using the SPAR Ontologies according to the new OpenCitations metadata document [1], and are published under a Creative Commons public domain dedication (CC0), so that others may freely build upon, enhance and reuse them for any purpose, without restriction under copyright or database law. We have described the new OpenCitations Corpus, and the new software developed by Silvio to create it, in [2].

OpenCitations is being continuously populated from the scholarly literature, and, as of 30th March 2017, has ingested the references from 123,989 citing bibliographic resources, and contains information about 5,307,857 citation links to 3,469,648 cited resources.

The whole OCC is now available for querying (via SPARQL), and for browsing by means of a very simple Web interface that shows only the data about bibliographic entities (e.g. Additional more user-friendly interfaces will be available in the coming months. The entire contents of the OpenCitations Corpus (OCC) are also archived every month as data dumps that are made available online through Figshare. Each dump comprises several zip archives, each containing either data or provenance information of a particular sub-dataset of the OCC.

Despite the fact that OpenCitations presently contains only a small proportion of global citation data, it is important to realize that, because of the very nature of scholarly citation, even this partial coverage includes citations of the most important papers in every biomedical field, these critical papers being characterized by the high number of their inward citation links.

[1] Silvio Peroni, David Shotton (2016). Metadata for the OpenCitations Corpus. figshare.

[2] Silvio Peroni, David Shotton, Fabio Vitali (2016). Freedom for bibliographic references: OpenCitations arise. Proceedings of 2016 International Workshop on Linked Data for Information Extraction (LD4IE 2016): 32-43.

Cite this article as: davidshotton, "Open Citations is dead. Long live OpenCitations.," in OpenCitations blog, 30/03/2017,

Open Citations Extension Project

I am pleased to announce that the JISC have funded an extension to the Open Citations Project to run from 1st August 2012 until 31st January 2013, during which we will review and revise the technology used to create the Open Citations Corpus, will update the content provided by PubMed Central, and will improve its presentation.  We will also prototype value-added services over the open citations data, to demonstrate their usefulness and to justify further funding that will permit a major expansion of the corpus in 2013 to include reference lists from subscription-access journals, including those from Nature Publishing Group, Science and other AAAS publications, and Oxford University Press.  In preparation for this, we will collaborate with CrossRef to determine how best to ingest reference data into the Open Citations Corpus in an on-going manner.

Full details are given in the Case for Support.  Watch this space!

Cite this article as: davidshotton, "Open Citations Extension Project," in OpenCitations blog, 16/07/2012,

Oxford University Press to support Open Citations

I am delighted to announce that Cathy Kennedy, OUP’s Senior Publisher for Journals, has just written to me as follows:

“Oxford University Press is delighted to support the Open Citation Corpus initiative in the interest of furthering and disseminating scholarship.  We are making the reference lists of articles in a number of journals we own available for inclusion in the Open Citations Corpus, and will be consulting further with our partner societies on extending the initiative to include their journals in due course”.

OUP thus becomes the third publisher of subscription-access journals, and the first university press, to declare its willingness to make the reference lists in journal articles publicly available.  Congratulations, OUP!

Are we starting to see the beginning of a sea change in attitudes?  Watch this space.

Cite this article as: davidshotton, "Oxford University Press to support Open Citations," in OpenCitations blog, 22/06/2012,

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,

Science joins Nature in opening reference citations

Hot on the heels of my announcement on Monday that Nature Publishing Group has agreed to open its articles’ reference lists, I am delighted to announce that Science Magazine, the American Association for the Advancement of Science’s premier global science weekly, together with the other AAAS journals Science Signalling and Science Translational Medicine, will also open their articles’ reference lists, and contribute the bibliographic citations contained within these lists as open linked data to an expanded Open Citations Corpus, where they will be freely available for everyone to use in whatever manner they choose.

Preparations to expand the corpus in this manner, by integration with the reference processing pipeline of the CrossRef Cited-By Linking service, will be undertaken over the last six months of this year, and incorporation of the references from the AAAS journals into the expanded Open Citations Corpus is planned to commence in the first half of 2013.

Now that the world’s two most presigeous scientific journals have declared their hands, it is time for other subscription-access publishers to step up and join them, for the reasons given in my earlier post.  I am presently in discussion with a small number of other major STM publishers, and hope to be able to make further announcements in the near future.  However, there are many other publishers with whom I have not yet been able to make personal contact.  Those who would also like to become ‘early adopters’ of the Open Citations publishing model should contact me at <>.

Cite this article as: davidshotton, "Science joins Nature in opening reference citations," in OpenCitations blog, 16/06/2012,

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 <>.

Like a kid with a new train set! Exploring citation networks

As part of the Open Citations Project, Alex Dutton recently completed a graphing plug-in for the Open Citations web site, that permits users to generate different kinds of graphs of citation networks by querying the Open Citation Corpus for a particular article, and either display the network of papers citing that article (input citations), papers cited by that article (output citations), or both.  These can be displayed on screen in the web browser in a variety of layouts, or conveniently downloaded in a number of useful formats.


Having survived the preparation and posting of the JISC Open Citations Project Final Blog Post last night, minutes before the midnight deadline, I’m now like a kid with a new train set, playing with this display tool and exploring the citation networks present in the Open Citation Corpus, something I have dreamed of doing for two years now.

Remember first that in the Open Citations Corpus we have some 200,000 citing articles – those within the Open Access Subset (OASS) of Pubmed Central – citing ~3.4 million papers out there in the big wide world, which are only recipients of citations.  The consequence of this limited corpus is that the majority of citation chains are of length one – from a paper in the OASS to a paper outside the OASS.  Not very interesting.  Add to this the fact that PubMed Central is new – over 90% of the papers in the Open Access Subset were published in the 21st Century, and 77% of them in the last 5 years.  Thus there are only a very few citation from articles within the OASS to other articles within the OASS.  That means that the maximum length of our citation chains, at present, to three or four on links the input side – a selected article may be cited by a chain of three or four other OASS articles, and three or four on the output side – the selected article may cite other OASS articles in addition to non-OASS articles, and these in turn will cite others.  However, in most cases, the citations chains are much shorter.

simple network
simple networkFigure 1. A simple citation network of input citation chain length of 2 links within the Open Citations Corpus, and an output chain length of 1 link – the selected article (red) receives citations from other OASS articles (green), and itself cites only articles outside the OASS (white).

Let’s start with something familiar – the article in PLoS Neglected Tropical Diseases by Reis et al. (2008) [1] that I used for our semantic publishing exemplar [2].  Its inward citation graph, limited to a citation chain length of two links, created by and copied from the Open Citations Project web site, looks like this:

input citations of Reis
Figure 2. The input citation network of Reis et al. (2008), limited to an citation chain length of 2 links.

I, of course, cited the Reis et al. (2008) paper [1] in our 2009 Adventures paper [3] that we based upon it, and also in my first paper on CiTO in 2010 [3], which also cites the Adventures paper.   Reis et al. (2008) is also cited by Fink et al. (2010) [4], who also cited our Adventures paper, and by Bourhy et al. (2010) [5], another PLoS Neglected Tropical Diseases paper in the OASS, which in turn is cited by Galloway and Levett (2010) [6], while our Adventures paper is also cited by Gerner and Nenadic (2010) [7].

The following image shows this graph as it was originally created within the Open Citations web page:

Input citations of Reis in web page
Figure 3. The same input citation network of Reis et al. (2008), as shown in the Open Citations web site.

Since Reis et al. (2008) has a reference list containing 52 references, its output citation graph is much more complex, even when limited to a citation chain length of 2, since several of its cited papers are also members of the Open Access Subset.  The following figure shows the whole output citation network a citation chain length of 2, which is too demagnified to be legible.

Citations by Reis
Figure 4. The outward citation network of Reis et al. (2008), limited to a citation chain length of 2 links.

The next figure shows a close-up of part of the previous diagram – the output citation network of Reis et al. (2008), again showing the Reis et al. (2008) paper in red, and one of the key papers it cites, Maciel et al. (2010) [8], a slightly earlier paper from the same research group, forming a second key node in the top right of the diagram.

Cited by Reis closeup
Figure 5. A close-up of a central portion of the outward citation network of Reis et al. (2008), limited to a citation chain length of 2 links.

Clearly, there is lots of information that can be extracted from these graphs, particularly when we display them in a tool like GraphViz that permits interactions with the data.  While the Open Citations web site simply displays such citation graphs created using one of several layout algorithms selected by the user, the raw data can also be downloaded in a variety of formats including GraphViz, GraphML and SVG, while the resulting network images can be downloaded in as PNG, JPEG and PDF images, and the underlying RDF metadata can be downloaded as RDF/XML. N-triples, Notation3 and Turtle.

Having used our new Open Citations web site and its network display interface for a short while, I am already aware of many shortcomings and limitations that we will attempt to improve upon in the next few days.  However, we would very much like to hear from you – as a user of the Open Citations web site – both to learn what you like about what we have done and to hear what you find to be shortcomings of the functionality and new features that you would like to see implemented, which we will record as user stories to input into our next round of development.  These can either be recorded as comments on this blog post, or can be e-mailed with the subject line “Open Citations web site” either to me <> or to Alex Dutton <>, who is the person who deserves all the credit for the present system.  We look forward to hearing from you.

[1]  Reis RB, Ribeiro GS, Felzemburgh RDM, Santana FS, Mohr S, Melendez SXTO, Queiroz A, Santos AC, Ravines RR, Tassinari WS, Carvalho MS, Reis MG, Ko AI (2008). Impact of environment and social gradient on Leptospira infection in urban slums. PLoS Negl Trop Dis 2(4): e228. doi:10.1371/journal.pntd.0000228.

[2] Shotton D, Portwin K, Klyne G, Miles A (2009). Adventures in semantic publishing: exemplar semantic enhancements of a research article. PLoS Comput Biol 5:e1000361. doi:10.1371/journal.pcbi.1000361.

[3] Shotton D (2010). CiTO, the Citation Typing Ontology. Journal of Biomedical Semantics  1 (Suppl. 1): S6. doi:10.1186/2041-1480-1-S1-S6.

[4]  Fink JL, Fernicola P, Chandran R, Parastatidis S, Wade A, Naim O, Quinn GB, Bourne PE (2010). Word add-in for ontology recognition: semantic enrichment of scientific literature.  BMC Bioinformatics 11:103. doi:10.1186/1471-2105-11-103.

[5]  Bourhy P, Collet L, Clément S, Huerre M, Ave P, Giry C, Pettinelli F, Picardeau M (2010). Isolation and Characterization of New Leptospira Genotypes from Patients in Mayotte (Indian Ocean). PLoS Negl Trop Dis 4(6): e724. doi:10.1371/journal.pntd.0000724.

[6]  Galloway RL, Levett PN (2010) Application and Validation of PFGE for Serovar Identification of Leptospira Clinical Isolates. PLoS Negl Trop Dis 4(9): e824. doi:10.1371/journal.pntd.0000824.

[7]  Gerner M, Nenadic G (2010). LINNAEUS: A species name identification system for biomedical literature. BMC Bioinformatics 11:85. doi:10.1186/1471-2105-11-85.

[8]  Maciel EAP, Carvalho ALF, Nascimento SF, Matos RB, Gouveia EL, Reis MG, Ko AI (2008). Household transmission of Leptospira infection in urban slum communities. PLoS Negl Trop Dis 2: e154. doi:10.1371/journal.pntd.0000154.

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