Citations as First-Class Data Entities: The OpenCitations Data Model

Requirements for citations to be treated as First-Class Data Entities

In my introductory blog post, I listed five requirements for the treatment of citations as first-class data entities.  The second of these requirements is that they must have metadata structured using a generic yet appropriately detailed data model.

To fulfil that requirement, OpenCitations is pleased to announce the publication on 13 February 2018 of the OpenCitations Data Model, v1.6 [1].  This replaces the previous version, v1.5.3, published on 13 July 2016.

The data model has been expanded and enhanced to improve the recording of publication dates, to include the treatment of citations as first-class data entities, and to permit the model’s adoption by third parties who may wish to use it to model their own citation data, or to prepare their citation data for publication in the OpenCitations Corpus (OCC).  To facilitate this, the document describing this data model is published under a Creative Commons Attribution 4.0 International license.

In addition to a change in the title from “Metadata for the OpenCitations Corpus” to “The OpenCitations Data Model”, and the use of the name “OpenCitations” (one token with two words in camel case) in place of “Open Citations” (with the space separating the two words), the substantive changes in the model from the previous version are as follows:

New class

A new class, Archival document, has been added as a subclass of bibliographic resource, to permit the model to be used for work on ancient manuscripts.

Publication dates

The mechanism for recording the publication dates of bibliographic resources has been improved, and now accepts the full date of publication (yyyy-mm-dd, if available), or the year plus the month of publication (yyyy-mm, if the full date is not available), or failing that just the year of publication (yyyy, as in the previous version of the data model).   In order to support this modification in the OWL mapping, prism:publicationDate is now used instead of fabio:hasPublicationYear.

Citations as first-class data entities

A new class of bibliographic entity, Citation, has been added to permit the description of citations as first-class data entities.  This class has been assigned sub-classes (e.g. Author self-citation) and properties (e.g. citation time span) to permit the description of citations in a manner helpful for bibliometric analysis.  These, and associated changes to CiTO, the Citation Typing Ontology, are described more fully in the previous blog post.

Virtual entities

The OpenCitations Data Model now permits the definition of virtual entities, i.e. bibliographic entities that are defined on-the-fly, only when they are requested (for example, by accessing their URLs). These are defined either by using data relating to non-virtual bibliographic entities that are already available within the OCC, or by using data that are themselves obtained on-the-fly from an external supplier (e.g. Wikidata).

This approach of using virtual RDF resources is optional, and is simply employed for storage efficiency, to avoid duplication of information within the OCC triplestore. As of January 2018, only one type of bibliographic entity is defined as a virtual entity, namely a citation (a members of the class Citation).

Such a virtual entity does not have the full provenance information normally associated with other bibliographic entities within the OCC, but it does have associated with itself the date of its creation and direct links both to the agent responsible for such creation and to the source data used in its construction.

Because we do not separately store these virtual entities within the Corpus triplestore, they cannot be directly queried by means of the OCC SPARQL end-point, neither are they stored within its data dumps. However, the data associated with an OCC virtual entity can be obtained by accessing its URL, which has form “https://w3id.org/oc/virtual/xyz”, clearly distinguishable from those URIs used for other (non-virtual) OCC bibliographic entities which have the form “https://w3id.org/oc/corpus/xyz”.  More details and examples are given in the Data Model document itself.

Additionally, for citations defined using Open Citation Identifiers (OCIs, described in a subsequent blog post), details of the cited and citing publications may be readily obtained by using the Open Citation Identifier Resolution Service at http://opencitations.net/oci.

Supplier prefixes

To enable citation data created by third parties to be incorporated within the OpenCitations Corpus, from February 2018 the OCC local identifiers for bibliographic resources now include a supplier prefix which clearly identifies the provenance of the data.  The prefix consists of a positive number (following the pattern “nnn”, where “nnn” is a string of numerals of variable length which includes no zeros), enclosed between two zeros (e.g. “0420”).

To ensure uniqueness of prefixes used by different suppliers, all organizations wishing to adopt the OpenCitations Data Model and to use it to create publicly available citation data, whether these are published in the OpenCitations Corpus or independently, must apply to OpenCitations for a unique supplier prefix, by sending an email to support@opencitations.net.  A list of already assigned supplier prefixes is available at https://github.com/opencitations/oci/blob/master/suppliers.csv.

The appropriate supplier prefix is combined with a unique numerical string that forms the ‘body’ of the identifier to create the local identifier used in OCC to identify an individual bibliographic resource.  OCC local identifiers for citations (as opposed to bibliographic resources) are constructed by combining the local identifiers for the citing and cited bibliographic resources, separating them with a dash.  Thus, for a citation between two bibliographic resources described in an external bibliographic database where they are each identified by an identifier having a unique numerical part, the OCC local identifiers for the citing and cited bibliographic resources are combined, separating them with a dash.

For example, the citation between citing Wikidata resource Q27931310 and cited Wikidata resource Q22252312 is given the OCC local citation identifier “01027931310-01022252312”, where “010” is the OCC supplier prefix (defined above) for Wikidata.  How these OCC local identifiers for citations are used to create Open Citation Identifiers is described in a separate blog post.

We commend the OpenCitations Data Model to anyone considering the storage of citation information, particularly if it is to be encoded in RDF, and we welcome contributions of citation data encoded using this model for publication within the OpenCitations Corpus.

Reference

[1]     Silvio Peroni, David Shotton (2018). The OpenCitations Data Model. Version 1.6. figshare. https://doi.org/10.6084/m9.figshare.3443876

Citations as First-Class Data Entities: Citation Descriptions

Requirements for citations to be treated as First-Class Data Entities

In my introductory blog post, I listed five requirements for the treatment of citations as first-class data entities.  The first of these requirements is that they must be definable in a machine-readable manner as a member of the class “Citation”, and describable using appropriate ontology terms.

This blog post describes recent additions to the OpenCitations Data Model, and to CiTO, the Citation Typing Ontology, that permit the required richer description of citations.

Changes to the OpenCitations Data Model

In the OpenCitations Data Model (OCDM), itself described in the following blog post, we have created the following new classes and properties that permit the descriptions of citations in richer ways that are appropriate for bibliometric research.  These changes have been inspired by the publications of Vincent Larivière, Ludo Waltman and their colleagues [1-3].

These new classes and properties and their definitions are described below:

New classes

  • Citation: a permanent conceptual directional link from the citing bibliographic resource to a cited bibliographic resource, created by the performative act of an author citing a published work that is relevant to the current work, typically made by including a bibliographic reference in the reference list of the citing work, or by the inclusion within the citing work of a link, in the form of an HTTP Uniform Resource Locator (URL), to the cited bibliographic resource on the World Wide Web.

The class Citation has sub-classes defining a particular type of citation.

  • Self-citation: a citation in which the citing and the cited entities have something significant in common with one another. Sub-classes include:
    • Affiliation self-citation: a citation in which at least one author from each of the citing and the cited entities is affiliated with the same academic institution.
    • Author network self-citation: a citation in which at least one author of the citing entity has direct or indirect co-authorship links with one of the authors of the cited entity.
    • Author self-citation: a citation in which the citing and the cited entities have at least one author in common.
    • Funder self-citation: a citation in which the works reported in the citing and the cited entities were funded by the same funding agency.
    • Journal self-citation: a citation in which the citing and the cited entities are published in the same journal.
  • Journal cartel citation: a citation from one journal to another journal which forms one of a very large number of citations from the citing journal to recent articles in the cited journal.
  • Distant citation: a citation in which the citing and the cited entities have nothing significant in common with one another over and beyond their subject matter.

New object properties

  • has citing document: The bibliographic resource which acts as source for the citation.
  • has cited document: The bibliographic resource which acts as target for the citation.

New data properties

  • has citation creation date:The date on which the citation was created. This has the same numerical value as the publication date of the citing bibliographic resource, but is a property of the citation itself. When combined with the citation time span, it permits that citation to be located in history.
  • has citation time span: The temporal characteristic of a citation, namely the interval between the publication date of the cited entity and the publication date of the citing entity.

Changes to CiTO, the Citation Typing Ontology

To complement these additions to the OpenCitations Data Model, and to permit these richer characteristics of citations to be encoded in RDF, we have additionally made the following changes to CiTO, the Citation Typing Ontology.

New classes

The class cito:SelfCitation has been renamed cito:AuthorSelfCitation, with an unchanged definition (“a citation in which the citing and the cited entities have at least one author in common”).

A new class cito:SelfCitation has been created, with same the more general definition as for this sub-class in the OCDM (“a citation in which the citing and the cited entities have something significant in common with one another”). In CiTO, this now includes five new sub-classes:

  • cito:AuthorSelfCitation
  • cito:JournalSelfCitation
  • cito:FunderSelfCitation
  • cito:AffiliationSelfCitation
  • cito:AuthorNetworkSelfCitation

with the definitions given above for these sub-classes in the OCDM.

New object properties

To complement the OCDM properties, we have within CiTO the following object properties:

  • cito:hasCitedEntity (“A property that relates a citation to the cited entity”) and
  • cito:hasCitingEntity (“A property that relates a citation to the cited entity”).

CiTO also has the following relevant object property:

  • cito:sharesPublicationVenueWith

with the sub-property cito:sharesJournalWith.

New data properties

To match the additions in the OCDM, we have added these new data properties to CiTO, which have the same definitions as those in the OCDM:

  • cito:hasCitationCreationDate
  • cito:hasCitationTimeSpan.

In addition, the class cito:AuthorNetworkSelfCitation is accompanied by the new data property:

  • cito:hasCoAuthorshipCitationLevel

which specifies the minimal distance that one of the authors of a citing entity has with regards to one of the authors of a cited entity according to their co-author network. For instance, a citation has a co-authorship citation level equal to 1 if at least one author of the citing entity has previously published as co-author with one of the authors of the cited entity. Similarly, we say that a citation has a co-authorship citation level equal to 2 if at least one author of the citing entity has previously published as co-author with someone who him/herself has previously published as co-author with one of the authors of the cited entity. And so on.

Describing a citation in RDF

Describing a citation between two articles in RDF as a simple link is straightforward but relatively uninformative:

<https://w3id.org/oc/corpus/br/1>
      cito:cites
          <https://w3id.org/oc/corpus/br/18> . 

The alternative RDF description of a citation as a first-class date entity could include the following triples (omitting any provenance information in this example), where br/1 and br/18 are the internal identifiers for the citing bibliographic resource and the cited bibliographic resource within the OpenCitations Corpus:

<https://w3id.org/oc/virtual/ci/1-18> a cito:Citation ;
     cito:hasCitingEntity <https://w3id.org/oc/corpus/br/1> ;
     cito:hasCitedEntity <https://w3id.org/oc/corpus/br/18> ;
     cito:hasCitationCreationDate "2016"^^xsd:gYear ;
     cito:hasCitationTimeSpan "P10Y"^^xsd:duration ;
     datacite:hasIdentifier <https://w3id.org/oc/virtual/id/ci-1-18> .

The meaning of “virtual” in the URI of this citation is explained in the following blog post about the OpenCitations Data Model.

The following diagram prepared by Silvio Peroni shows the semantic relationships for a citation currently handled by the OpenCitations Corpus (omitting the sub-classes of the class cito:Citation).  Explanation of OCI, the Open Citation Identifier, is given in a subsequent post.

References

[1]     Matthew L. Wallace, Vincent Larivière and Yves Gingras (2012. A Small World of Citations? The Influence of Collaboration Networks on Citation Practices.  PLoS ONE 7(3): e33339. https://doi.org/10.1371/journal.pone.0033339

[2]     Philippe Mongeon, Ludo Waltman and Sarah de Rijcke (2016). What do we know about journal citation cartels? A call for information.  CWTS blog post. Available at https://www.cwts.nl/blog?article=n-q2w2b4

[3]       Ludo Waltman and Caspar Chorus (2016). Journal self-citations are increasingly biased toward impact factor years. CWTS blog post. Available at https://www.cwts.nl/blog?article=n-q2x264

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