One year of the OpenCitations Corpus Releasing RDF-based scholarly citation data into the Public Domain

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Reference lists from academic articles are core elements of scholarly communication that permit the attribution of credit and integrate our independent research endeavours. Hitherto, however, they have not been freely available in an appropriate machine-readable format such as RDF and in aggregate for use by scholars. To address this issue, one year ago we started ingesting citation data from the Open Access literature into the OpenCitations Corpus (OCC), creating an RDF dataset of scholarly citation data that is open to all. In this paper we introduce the OCC and we discuss its outcomes and uses after the first year of life.


Citations are the links that knit together our scientific and cultural knowledge

However often they are enclosed in closed citation indexes thought only for human consumption and/or accessible only by paying significant fees

The OpenCitations Corpus (OCC, is a LOD repository of CC0 citation data described with SPAR Ontologies (

It provides >11M citation links from ~260,000 citing articles to ~6M cited resources + provenance information

Ingestion workflow

We developed several scripts for implementing the ingestion workflow that populates the OpenCitations Corpus

All the software is available on the OpenCitations GitHub repository and released as open source code with the ISC License

These scripts implement a live and iterative process

  • Live: it's working while I'm speaking. It doesn't sleep, never. It's like a sentient, relentless, fast zombie – watch out!

  • Iterative: the ingestion workflow continuously calls several external APIs to obtain new reference lists and clean metadata of the citing and cited papers

External APIs

At present, all the reference lists are taken by processing the XML sources of the papers in the PubMed Central Open Access subset

Europe PubMed Central API for retrieving the XML sources

  • We ask for the most recent papers first

  • Citing papers includes articles published in 2016 and 2017

  • There are 1.75M OA articles available in PubMed, according to their API. We have harvested 15% so far...

Crossref APIs to obtain additional information (title, authors, venues, etc.) about citing/cited papers, and then call the ORCID APIs to obtain ORCIDs of the authors

Data model

Available at and implemented in the OpenCitations Ontology

Resources in the OCC

Entity typeWhat it describesCount in the OCC
Bibliographic resource (br)Conference papers, book chapters, journal articles, academic proceedings, books, journals, etc. ~7.3M
Resource embodiment (re)Digital vs. print, first and ending pages, etc. ~4.3M
Bibliographic entry (be)Textual content of a reference in a reference list ~10.5M
Resposible agent (ra)Given name, family name and ORCID of the agent involved ~22.5M
Agent role (ar)Author, publisher, etc. ~28.6M
Identifier (id)DOI, PubMed ID, PubMed Central ID, ORCID, ISSN, etc.~15.1M

Data statementsProvenance statements

Entities in the OCC

Entity typeKinds of entityEntities with identifiers
Bibliographic resource

Academic Proceedings: 13,804

Book: 47,824

Book Chapter: 68,798

Book Series: 15

Data File: 4,155

Expression Collection: 10,321

Journal: 27,373

Journal Article: 4,481,224

Journal Issue: 1,029,369

Journal Volume: 365,286

Part: 6

Proceedings Paper: 28,125

Reference Book: 66

Reference Entry: 7,917

Report Document: 1,946

Series: 155

Specification Document: 600

Thesis: 768

Generic kind: 1,281,627

DOI: 4,697,326

ISBN: 82,329

ISSN: 38,359

PMCID: 1,280,229

PMID: 3,954,683

URL: 4,835,585

Responsible agent -

ORCID: 240,989

URL: 5,642


DOI: 4,697,326

ISBN: 82,329

ISSN: 38,359

ORCID: 240,989

PMCID: 1,280,229

PMID: 3,954,683

URL: 4,841,227


FAIR citation data

More details:

Accessing OCC data

Dump views and downloads

Usage of the OCC

Some known adopters: Wikidata, OpenAIRE, LOC-DB, eLife, Ontotext, independent researchers (Anna Kamińska, Daniel Himmelstein, Thiago Nunes and Daniel Schwabe)

Website accesses

Use by country: United States (33.7%), France (31.3%), Bulgaria (14.3%)


OpenCitations Blog on Wordpress at


OpenCitations Twitter account at


The Initiative for Open Citations (I4OC, is a collaboration between scholarly publishers, researchers, and other interested parties to promote the unrestricted availability of scholarly citation data

Goal: promote the availability of data on citations that are available in machine-readable formats, decoupled from the source they have been extracted, freely accessible and usable

What happened: before I4OC launch (6 April 2017), 1% of publications in Crossref with open references


Current and future infrastructure

It's a virtual machine! It doesn't exist

DELL PowerEdge R730xd V4, 512GB RAM, 22TB HD + 30 Raspberry Pi 3

We have recently received a small grant from the Sloan Foundation for one year's salary for a postdoc to develop new user interfaces, and new hardware to enhance the OCC performance - from 8M citations per year to 240M

Data are coming

END One year of the OpenCitations Corpus