Characterizing CiTO properties – the CiTO Functions Ontology
CiTO, the Citation Typing Ontology, contains 41 object properties that can be used to characterize the nature of a bibliographic citation linking the citing paper to the citing paper, plus an equal number of their inverse properties. Some of these are also appropriate for the characterization of data citations, as described here.
These can be classified into factual properties (e.g. cito:usesDataFrom) and rhetorical properties, with the latter being sub-grouped into positive properties (e.g. cito:supports), neutral properties (e.g. cito:reviews) and negative ones (e.g. cito:critiques).
These sub-groupings are described in a new extension ontology, the CiTO Functions Ontology available from http://www.essepuntato.it/2013/03/cito-functions.
However, despite this extensive list of CiTO properties, there will be situations in which the purpose of making a citation cannot be adequately expressed using these CiTO properties. In such situations, it is now possible to use the new Open Annotation Data Model (http://www.openannotation.org/spec/core/) to define the reason for, or the nature of, the citation.
New CiTO class and properties to use with the Open Annotation Data Model
To make this straightforward, we require a mechanism that permits the citation itself to objectified, so that it can become the object of an RDF triple defined using a URI. To facilitate this, we have added a new class to the most recent version of CiTO, CiTO v2.6.2, that describes the citation itself:
cito:CitationAct A citation act is a performative act of making a citation from a citing entity to a cited entity, typically instantiated by the inclusion of a bibliographic reference or a data reference in the reference list of the citing entity.
To complement this class, three new object properties have been added to CiTO v2.6.2:
cito:hasCitationCharacterization A property that links a cito:CitationAct to its characterization made by using a CiTO citation characterization property such as cito:confirms. cito:hasCitingEntity A property that relates a cito:CitationAct to the entity that contains that citation. cito:hasCitedEntity A property that relates a cito:CitationAct to the entity that is the target of that citation.
Adopting the same philosophy as used for the other CiTO object properties, we have avoided adding domain and range restrictions to these properties, enabling them to be used in circumstances that we do not foresee.
Reifying citation statements
This new CiTO class cito:CitationAct, and its accompanying new object properties cito:hasCitingEntity, cito:hasCitationCharacterization and cito:hasCitedEntity, can be employed to reify direct citation act statements made using the CiTO citation object property cito:cites or one of its sub-properties. For example, the following RDF statement:
:paperA cito:extends :paperB .
can be alternatively described as follows:
:thisCitation a cito:CitationAct ; cito:hasCitingEntity :paperA ; cito:hasCitationCharacterization cito:extends ; cito:hasCitedEntity :paperB .
This usage involved OWL2 punning, whereby a CiTO object property, such as the aforementioned cito:extends, is used as the object of the OWL assertion:
:thisCitation cito:hasCitationCharacterization cito:extends .
Using such OWL2 punning (described at http://www.w3.org/TR/2009/WD-owl2-new-features-20090611/#F12:_Punning), the CiTO object property is considered as a proper named individual of the class owl:Thing.
Such reification of citation acts can be very useful, since it permits one to combine these CiTO properties with other vocabularies, or to handle situations in which none of the citation characterizations available in CiTO are applicable. Such situations can be resolved by the creation of a user-defined citation characterization, for example by using the Open Annotation Data Model.
Introducing the Open Annotation Data Model
Use of the Open Annotation Data Model is well described by Robert Sanderson in his SlideShare presentation at http://www.slideshare.net/azaroth42/oai8-openanno. In this model, an annotation is described as a member of the class oa:Annotation, which has a body containing the annotation itself defined by oa:hasBody, and an annotation target (the thing to which the annotation relates) defined by oa:hasTarget, as shown in the following figure.
Using the Open Annotation Data Model to characterize citations
In our case, the target of the annotation is a member of the class cito:CitationAct, while the body, i.e. the textual content of the annotation itself, is described using the W3C Content Vocabulary (http://www.w3.org/TR/Content-in-RDF10/) as a member of the class cnt:ContentAsText, which the property cnt:chars relates to the text string actually providing the annotation.
As shown in the figure, an OA annotation can be further characterized by the motivation for making the annotation, defined by oa:isMotivatedBy. In our case, the appropriate motivation is oa:commenting, an instance of the class oa:Motivation.
Thus characterization of a bibliographic citation using the Open Annotation Data Model can take the following form:
:thisAnnotation a oa:Annotation; oa:motivatedBy oa:commenting ; oa:hasBody [ a cnt:ContentAsText ; cnt:chars "I'm citing that paper because it initiated this whole new field of research." ] ; oa:hasTarget [ a cito:CitationAct; cito:hasCitingEntity :PaperA ; cito:hasCitationEvent cito:cites ; cito:hasCitedEntity :PaperB ] .
Simple and flexible! We commend use of the Open Annotation Data Model to make such “free-hand” citation characterizations.