The Sloan Foundation funds OpenCitations

The OpenCitations Enhancement Project funded by Sloan

The Alfred P. Sloan Foundation, which funds research and education in science, technology, engineering, mathematics and economics, including a number of key technology projects relating to scholarly communication, has agreed to fund The OpenCitations Enhancement Project, a new project to develop and enhance the OpenCitations Corpus.

As readers of this blog will know, the OpenCitations Corpus is an open scholarly citation database that freely and legally makes available accurate citation data (academic references) to assist scholars with their academic studies, and to serve knowledge to the wider public.


The OpenCitations Enhancement Project, funded by the Sloan Foundation for 18 months from May 2017, will make the OpenCitations Corpus (OCC) more useful to the academic community both by significantly expanding the volume of citation data held within the Corpus, and by developing novel data visualizations and query services over the stored data.

At OpenCitations, we will achieve these objectives in the following ways:

(a) By establishing a new powerful physical server to handle the Corpus data and offer adequate performance for query services.

(b) By increasing the rate of data ingest into the Corpus, by integrating with server 30 small data-ingest computers, Raspberry Pi 3Bs, working in parallel to harvest references, thus increasing the current rate of corpus data ingest some thirty-fold to about half a million citation links per day.

(c) By employing a post-doctoral computer science research engineer specifically to develop information visualisation interfaces and sense-making tools that will both provide smart ways of envisaging and comprehending the citation data stored within the OpenCitations Corpus, and will also ease the task of manual curation of the OCC.


This post-doctoral appointment will start in the autumn of 2017, once the new hardware has been commissioned and programmed. We seek a highly intelligent, skilled and motivated individual who is an expert in Web Interface Design and Information Visualization, and who can demonstrate a commitment to increasing the openness of scholarly information. A formal advertisement for this post, which will be held at the University of Bologna in Italy under the supervision of Dr Silvio Peroni, will be published in the near future. In the mean time, individuals with the relevant skills and background who would like to express early interest in joining the OpenCitations team in this role should contact him by e-mail to <>.

Expected Outcomes

By the end of the OpenCitations Enhancement Project, we will have harvested approximately 190 million citation links obtained from the reference lists of about 4.4 million scholarly articles (~15% of Web of Science’s coverage). In this way, in a significant initial step towards the comprehensive literature coverage we seek for the OCC, we will establish the OpenCitations Corpus as a valuable and persistent free-to-use global scholarly on-line Linked Open Data service.

In so doing, we aim at empower the global community by liberating scholarly citation data from their current commercial shackles, publishing such data with a Creative Commons CC0 Public Domain Dedication that will enable novel third-party services to be built over them.

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Querying the OpenCitations Corpus

OpenCitations makes available a SPARQL endpoint for querying the data included in the OpenCitations Corpus. While several queries are possible according to the model described in the website (and, with more details, in the official metadata document of the Corpus), we have received some requests by users of the service for exemplar queries. We have chosen two of them, which are particularly relevant with regard to the work that has been done in the past months by the Initiative for Open Citations – that we have already introduced in another blog post.

Query: return all the papers (including their titles) citing the article with DOI “10.1038/227680a0”.

PREFIX cito: <>
PREFIX dcterms: <>
PREFIX datacite: <>
PREFIX literal: <>
SELECT ?citing ?title WHERE {
  ?id a datacite:Identifier ;
    datacite:usesIdentifierScheme datacite:doi ;
    literal:hasLiteralValue "10.1038/227680a0" .
    datacite:hasIdentifier ?id ;
    ^cito:cites ?citing .
  ?citing dcterms:title ?title

Query: return all the papers cited by the bibliographic resource “br/4186” included in the OCC, including the text of bibliographic references used in “br/4186” for making the citations and the titles of the cited papers.

PREFIX cito: <>
PREFIX dcterms: <>
PREFIX biro: <>
PREFIX frbr: <>
PREFIX c4o: <>
SELECT ?cited ?cited_ref ?title WHERE {
  <> cito:cites ?cited .
    <> frbr:part ?ref .
    ?ref biro:references ?cited ;
      c4o:hasContent ?cited_ref 
  OPTIONAL { ?cited dcterms:title ?title }
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The Initiative for Open Citations

OpenCitations are pleased to announce the launch of the Initiative for Open Citations (I4OC) , a fresh momentum in the scholarly publishing world to open up data on the citations that link research publications.  OpenCitations are proud to be a founder of I4OC, and we encourage those remaining publishers whose journal article reference lists are still closed to embrace this sea change in attitude towards open citation data. The other I4OC founding organizations are Wikimedia Foundation, PLOS, eLife, DataCite, and the Centre for Culture and Technology at Curtin University,

Until recently, the vast majority of citation data were not openly available, even though all major publishers freely share their article metadata through Crossref. Before I4OC started, only about 1% of the reference data deposited in Crossref were freely available. Today that figure has jumped to 40% [1].


In recent months, following earlier indications of willingness reported in this blog, several publishers have made the decision to release these metadata publicly, including the American Geophysical Union, Association for Computing Machinery, BMJ, Cambridge University Press, Cold Spring Harbor Laboratory Press, EMBO Press, Royal Society of Chemistry, SAGE Publishing, Springer Nature, Taylor & Francis, and Wiley. These publishers join other publishers who have been opening their references through Crossref for some time. The full list of scholarly publishers now opening their reference data via Crossef is given in [2].

These decisions stem from discussions that have been taking place since a call-to-action to open up citations was made by Dario Taraborelli of the Wikimedia Foundation at the 2016 OASPA Conference on Open-Access Publishing. The creation of I4OC was spearheaded by Jonathan Dugan, Martin Fenner, Jan Gerlach, Catriona MacCallum, Daniel Mietchen, Cameron Neylon, Mark Patterson, Michelle Paulson, Silvio Peroni, myself and Dario Taraborelli. The purpose of I4OC is to coordinate these efforts and to promote the creation of a comprehensive, freely-available corpus of scholarly citation data.


Such a corpus will be valuable for new as well as existing services, and will allow many more interested parties to explore, mine, and reuse the data for new knowledge. The key benefits that arise from a fully open citation dataset include:

  1. The establishment of a global public web of linked scholarly citation data to enhance the discoverability of published content, both subscription access and open access. This will particularly benefit individuals who are not members of academic institutions with subscriptions to commercial citation databases.
  2. The ability to build new services over the open citation data, for the benefit of publishers, researchers, funding agencies, academic institutions and the general public, as well as to enhancing existing services.
  3. The creation of a public citation graph to explore connections between knowledge fields, and to follow the evolution of ideas and scholarly disciplines.


The Internet Archive, Mozilla, the Wellcome Trust, and twenty eight other projects and organizations have formally put their names behind I4OC as stakeholders in support of openly accessible citations. The full list of stakeholders is given in [3].


Dario Taraborelli, Head of Research at the Wikimedia Foundation, said:

“Citations are the foundation for how we know what we know. Today, tens of millions of scholarly citations become available to the public with no copyright restriction. We look forward to more organizations joining this initiative to release, and build on these data.”

Liz Ferguson, VP Publishing Development, Wiley, said:

“Wiley is delighted to support I4OC by opening our citation metadata via Crossref. Collaborating with other publishers further contributes to sustainable and standardized infrastructure that will benefit the research community. We are particularly excited by the potential to expose networks of research that would otherwise lie hidden or take years to discover.”

Robert Kiley, Head of Open Research at the Wellcome Trust, said:

“The open availability of citation data will help all funders better evaluate the research they fund. The progress that I4OC has made is an essential first step and we encourage all publishers to publicly share this data.”

Mark Patterson, Executive Director of eLife, said:

“It’s fantastic to see the interest that’s being shown by so many publishers in making their reference list metadata publicly available. We hope that this new momentum will encourage all publishers to follow suit, and that new services and tools can be built around these open data.”

Catriona MacCallum Advocacy Director, PLOS, said:

“Creating an open database of citations will allow researchers to perform independent analyses of how scientific ideas are communicated through article citations, and a transparent way of tracking the influence of particular articles. By opening up these metadata via Crossref, publishers are providing a vital contribution to Open Science.”

Future growth

Many other publishers have expressed interest in opening up their reference data. They can do this very easily via Crossref, with a simple email to requesting they turn on reference distribution for all their DOI prefixes. This is required even for publishers of open access articles, since by default references submitted to the Crossref Cited-By Linking service are closed, as previously explained here.  I4OC will provide regular updates on the growth of the public citation corpus, how the data are being used, additional stakeholders and participating publishers as they join, and as new services are developed.

I4OC and OpenCitations

Through the efforts of I4OC, scholarly citation data will be increasingly available to any interested party through all of Crossref’s Metadata Delivery Services, including the REST API and bulk metadata dumps. From this open source, OpenCitations will progressively import the citation data into the OpenCitations Corpus, describe them using the SPAR Ontologies according to the OCC metadata model, and make them available in RDF under a Creative Commons public domain dedication as Linked Open Data.  Potential users should be aware that is will take some considerable time before all the new citation data now available via the Crossref API are ingested into the OpenCitations Corpus.

I4OC links


[1] 40% is the percentage of publications with open references out of the total number of publications with reference metadata deposited with Crossref. As of March 2017, nearly 35 million articles with references are deposited with Crossref.

[2] Full list of publishers now making their citation data open via Crossref.

[3] Full list of I4OA supporting stakeholder organizations.

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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.

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Three publications describing the Open Citations Corpus

Last September, I attended the Fifth Annual Conference on Open Access Scholarly Publishing, held in Riga, at which I had been invited to give a paper entitled The Open Citations Corpus – freeing scholarly citation data.  A recording of my talk is available here, and my PowerPoint presentation is separately available here.  My own reflections on the major themes of the conference are given in a separate Semantic Publishing Blog post.

While in Riga preparing to give that talk about the importance of open citation data, I received an invitation from Sara Abdulla, Chief Commissioning Editor at Nature, to write a Comment piece for their forthcoming special issue on Impact.  My immediate reaction was that this should be on the same theme, an idea to which Sara readily agreed.  The deadline for delivery of the article was 10 days later!

As soon as the Riga conference was over, I first assembled all the material I had to hand that could be relevant to describing the Open Citations Corpus (OCC) in the context of conventional access to academic citation data from commercial sources.  That gave me a raw manuscript of some five thousand words, from which I had to distil an article of less than 1,300 words.  I then started editing, and asked my colleagues Silvio Peroni and Tanya Gray for their comments.

The end result, enriched by some imaginative art work by the Nature team, was published a couple of weeks later on 16th October [1], and presents both the intellectual argument for open citation data, and the practical obstacles to be overcome in achieving the goal of a substantial corpus of such data, as well as giving a general description of the Open Citations Corpus itself and of the development work we have planned for it.

Because of the drastic editing required to reduce the original draft to about a quarter of its size, all material not crucial to the central theme had to be cut.  I thus had the idea of developing the original draft subsequently into a full journal article that would include these additional themes, particularly Silvio’s work on the SPAR ontologies described in this Semantic Publishing Blog post [2], Tanya’s work on the CiTO Reference Annotation Tools described in this Semantic Publishing Blog post, and a wonderful analogy between the scholarly citation network and Venice devised by Silvio.  I also wanted to give authorship credit to Alex Dutton, who had undertaken almost all of the original software development work for the OCC.  For this reason, instead of assigning copyright to Nature for the Comment piece, I gave them a license to publish, retaining copyright to myself so I could re-use the text.  I am pleased to say that they accepted this without comment.

Silvio and I then set to work to develop the draft into a proper article.  The result was a ten-thousand word paper submitted to the Journal of Documentation a week before Christmas [3].  We await the referees’ comments!


[1]     Shotton D. (2013).  Open citations.  Nature 502: 295–297. doi:10.1038/502295a.

[2]     Peroni S and Shotton D (2012). FaBiO and CiTO: ontologies for describing bibliographic resources and citations. Web Semantics: Science, Services and Agents on the World Wide Web. 17: 33-34. doi:10.1016/j.websem.2012.08.001.

[3]    Silvio Peroni, Alexander Dutton, Tanya Gray, David Shotton (2015). Setting our bibliographic references free: towards open citation data. Journal of Documentation, 71 (2): 253-277.; OA at

This is the main article about OpenCitations, which includes several background information and the main ideas and works supporting the whole project, the Corpus, and some possible future developments in terms of new kinds of data to be included, e.g. citation functions.


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Open Citations – Doing some graph visualisations

Ongoing work on the Open Citations extensions project is now reaching the point of visualising – at very much a prototype level at this stage – the outputs of our earlier efforts to import and index the PubMed Central Open Access subset and arXiv.

Earlier in this project I asked David to specify a list of questions that he thought researchers might hope to answer by querying our Open Citations Corpus; the aim was to use these questions to guide our developments, in the hope of providing a striking interface that also did something useful – there are too many visualisations of data that look very pretty but that do not actually add much to the data. So, considering that list of questions and how one might visualise the data to not only a pretty but functional end, I set myself the following problem:

Identify what it is in a dataset that is not easy to find in a textual representation, and make it useful for search

Based on our earlier text search demonstrator the answer pretty soon became – of course – interactions; whilst the properties of a result object are obvious in a textual result set, the interactions between those objects are not – and sometimes, it is the interactions that one wishes to use as search parameters.

What I did

Having found my purpose, I set about applying the superb D3.js library to the problem, using it to draw SVG representations of elasticsearch query results directly into the browser. After testing with a number of different result layouts, I settled upon using a zoomable-pannable force-directed network graph and combined it up with some code from my PhD work to build in some connections on the fly. This is as mentioned earlier still a work in progress, but results so far are pretty good.

Take the image above, for example: this is a static representation of the interactions between David Shotton and all other authors (purple) with whom he has published an article (green) in the PMC OA subset. The red dots are the journals these articles are in, and the brown are citations. As a static image this could be fairly informative when marked up with appropriate metadata, and it does look quite nice; but, more than that, it can act as part of a search interface to enable a much improved search experience.

So far, the production of a given image also reduces the result set size; so whilst viewing the above image, the available suggestion dropdowns are automatically restricted to the subset of values relevant to the currenlty displaying image – dropdown suggestions are listed in order of popularity count, then upon typing one letter they switch to alphabetical, and with multiple letters they become term searches. By typing in free text search values or choosing suggestions, this visual representation of the current subset of results combined with the automated restriction of further suggestions should offer a simple yet powerful search experience. It is also possible to switch back to “list” view at any time, to see the current result set in a more traditional form. Further work – described below – will bring enhancements that add functionality to the elements of the visualisation too.

Try it

Similarly to the search result list demonstrator, it is possible to embed the visual search tool in any web page. However, as it looks better with full screen real estate I have saved that particular trick for the time being, and simply made it available at

Now before you rush off to try it, given the prototype state, you will need some pointers. Taking the above image as example once more, in order to reproduce it, do the following:

  • Use a modern browser – Chrome renders javascript the fastest – on a reasonably decent machine to access – a large screen resolution would be particularly nice
  • Choose authors as a search suggestion type
  • Start typing Shotton – click on Shotton David when it appears in the list
  • (If the error where it appears to return all results again appears – described below – just keep going)
  • tick the various display options to add authors, journals and citations objects to the display

The next step would be to click an author or other entity bubble then choose to add that to the search terms, or start a new search based on that bubble or perhaps a subset of the returned bubbles; however this is all still in development.

For a more complex example, try choosing keywords then type Malaria. Once displayed, increase resultset size to 400 so they are all displayed. Then try selecting the various authors, journals and citations tickboxes to add those objects; try increasing the sizes to see how many you can get before your computer melts… On my laptop, asking for more than about 1000 of each object results in poor performance. But here is an example of the output – all 383 articles with the Malaria keyword in the PMC OA, showing all 70 journals in which they are published, with links to the top 100 authors and citations. Which journal do you think is the large purple dot in the middle?

Outstanding issues


  • Numerous buttons have no action yet – clear / help / prev / next / + search / labels. Once these and other search action buttons are added, the visualisation can become a true part of the search experience rather than just a pretty picture
  • Searches are sent asyncrhonously and occasionally overlap, resulting in large query result sizes overwriting smaller ones. This needs a delay on user interactions added.
  • Some objects should become one – for example some citations are to the same article via both DOI and PMID, and some citations are also open access articles in our index, so they shold be linked up as such.
  • There is as yet no visual cue that results are still loading, so it feels a bit in limbo. Easy fix.
  • Some of the client-side processing can be shifted to the backend (already in progress)
  • The date slider at the bottom is twitchy and needs smoother implementation and better underlying data (see below)

Data quality

Apart from the above technical tasks, we will need to re-visit our data pipeline in order to answer more of the questions set by David. For example we have very little affiliation data at present, and we are also missing a large amount of date information. Also some data cleaning is necessary – for example, keywords should all be lowercased to ensure we do not have subsets due solely to capitalisation. There are also certain types of data that we have no idea about as yet – for example author location, h-index, ORCID. However, this is all as to be expected at this stage, and overall our ability to so easily spot these issues shows great progress.

More to come

There is still work to be done on this graph interface, and in addition, we have some more demonstrators on the way too. In combination with the work on improving the pipeline and data quality, we should soon be able to perform queries that will answer more of our set questions – then we will identify what needs done next to answer the remaining ones!


David Shotton writes: This productive collaboration between Cottage Labs and the Open Citations Corpus came to an end when Jisc funding ran out.  The corpus has more recently been given a new lease of life, as described here, with a new instantiation named OpenCitations hosted at the Department of Computer Science and Engineering of the University of Bologna, with Silvio Peroni as Co-Director.

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Open Citations Corpus Import Process

As part of the Open Citations project, we have been asked to review and improve the process of importing data into the Open Citations Corpus, taking the scripts from the initial project as our starting point.

The current import procedure evolved from several disconnected processes and requires running multiple command line scripts and transforming the data into different intermediate formats. As a consequence, it is not very efficient and we will be looking to improve on the speed and reliability of the import procedure. Moreover, there are two distinct procedures depending on the source of the data (arXiv or PubMed Central); we are hoping to unify the common parts of these procedures into a single process which can be simplified and normalised to improve code re-use and comprehensibility.

The Workflow

As PubMed Central provides an OAI-PMH feed, this could be used to retrieve article metadata, and for some articles, full text. Using this feed, rather than an FTP download (as used currently) would allow the metadata import for both arXiv and PubMed Central to follow a near-identical process, as we are already using the OAI-PMH feed for arXiv.

Also, rather than have intermediate databases and information stores, it would be cleaner to import from the information source straight into a datastore. The datastore could then be queried, allowing matches and linking between articles to be performed in situ. The process would therefore become:

  1. Pull new metadata from arXiv (OAI-PMH) and PubMed Central (OAI-PMH) and insert new records into the Open Citations Corpus datastore
  2. Pull new full-text from arXiv and PubMed Central, extract citations, and match with article data in Open Citations server, creating links between these references and the metadata records for the cited articles. Store unmatched citations as nested records in the metadata for each article.
  3. On a scheduled basis (e.g. nightly), review each existing article’s unmatched citations and attempt to match these with existing bibliographic records of other articles.

In outline, this looks like this:

The Datastore

Neo4J is currently used as the final Open Citations Corpus datastore for the arXiv data, by the Related Work system. We propose instead to use BibServer as the final datastore, for its flexibility and scalability, and suitability for the Open Citations use cases.

The Data Structure

The data stored within BibServer as BibJSON will be a collection of linked bibliographic records describing articles. Associated with each record and stored as nested data will be a list of matched citations (i.e. those for which the Open Citations Corpus has a bibliographic record), a list of unmatched citations, and a list of authors.

Authors will not be stored as separate entities. De-coupling and de-duplicating authors and articles could form the basis of a future project, perhaps using proprietary identifiers (such as ORCHID, PubMed Author ID or arXiv Author ID) or email addresses, but this will not be considered further in this work package.

Overall Aim

The overall aim of this work is to provide a consistent, simple and re-usable import pipeline for data for the Open Citations Corpus. In the fullness of time we’d expect it to be possible to add new data sources with minimal additional complexity. By using an approach whereby data is imported into the datastore at as early a stage as possible in the import pipeline, we can use common tools for extracting, matching, deduplicating citations; the work for each datasource, then, is just to convert the source data format into BibJSON and store it in BibServer.


David Shotton writes: This productive collaboration between Cottage Labs and the Open Citations Corpus came to an end when Jisc funding ran out.  The corpus has more recently been given a new lease of life, as described here, with a new instantiation named OpenCitations hosted at the Department of Computer Science and Engineering of the University of Bologna, with Silvio Peroni as Co-Director.

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