Relationships between DOIs and other objects

Online publications and physical print publications with a digital fingerprint often do not exist as a single content item. Many parts comprise electronic publications which are often produced, curated, and published by different organizations and separate activities.  A complete representation of a particular work requires interlinking this assemblage of parts in a navigable infrastructure that allows both readers and machines to consume the entirety of a specific intellectual property given the distributed locations of its parts.

Reference linking was the first service established by Crossref to address this need at the dawn of on-line scholarly publishing. Connecting the articles, and other content, identified in the bibliography, using the actionable DOI created a navigable citation network. Since then, Crossref has introduced other interlinking services that address specific types of relationships. Crossmark supports the connection of 'updates' which have a material effect on the original work, for example in the most extreme a retraction. Funding data supports identifying the organization that financially supports the research behind a specific publication. Components allow for the assignment of DOIs to the elemental parts of a publication (figures, tables, images) which may lead to their reuse. These and other Crossref services create relationships between content items; however, they share two characteristics that restrict their ability to define relationships.

1. Both items involved in a relationship must be identified by Crossref assigned DOIs.
2. The  types of relationships are dictated by the mission of the specific Crossref service.

Modified and new services developed in response to these two limitations.

1. Allow non Crossref DOIs to be deposited in an item's (article/chapter/paper) list of citations.
2. Support the creation of general typed relations between items having a Crossref DOI and other content items represented by a variety of identifiers.

Declaring relationship types

The nature of possible relationship types between content items can be as varied as the nature of the items themselves.  Constructing an orderly mapped network of content requires the use of a controlled vocabulary  that defines these relationships. Crossref's services achieve this in both an implicit approach where the relation type is a function of a specific service and is declared in the structure of the deposited XML and in an explicit approach where the relation type is selected as a value within the deposited metadata.

1. Reference linking and Cited-by: Implicitly creates  'cites' and 'isCitedBy' relations between a content item and the items in its bibliography.
2. Crossmark:  Explicit creation of update relations between an item and other items that material effect it (ex. a retraction).
3. Funding data:  Implicit creation of 'isFundedBy' and 'hasAward' relations between an item and the funding source that supported the underlying research
4. Linked clinical trials: Implicit creation of a 'belongsTo' relationship between and item and a registered clinical trial.
5. Components:  Implicit creation of a 'isChildOf' relationship between an item and its elemental parts that are assigned their own DOI (limited parent relation typing).
6. General typed relations: Explicitly typed relation between an item having a Crossref DOI and an item having one of several possible identifiers

Recommended relationship types for associated research objects

Comment	"hasComment"
Dataset generated as part of research results	"isSupplementedBy"
Dataset produced by a different set of researchers or previously published	"references"
Edited volume republication	"isVersionOf"
Material	"hasRelatedMaterial"
Peer review	"hasReview"
Preprint	"hasPreprint"
Protocol	"hasRelatedMaterial"
References	"references"
Software	"isCompiledBy"
Translation	"isTranslationOf"
Version	"isVersionOf"

General typed relations

This service allows for the creation of a typed relationship between an item having a Crossref DOI and another content item. The other item may be represented by another Crossref DOI, a DOI from some other Registration Agency, or an item not identified with a DOI. When DOIs are used the deposit process will fail if the DOI does not exist. Non-DOI identifiers are not verified.

When DOIs are used a bidirectional relation is automatically created when a relation is created in the deposit of one item in a pair. The DOI deposited with metadata creating the relation is said to be the claimant, the other item does not need to have its metadata directly contain the relationship.

Why do this?

Creating relationships  helps build a mapping of scholarly objects as described in Jennifer Lin's posting "Crossref & the Art of Cartography".  Expressing these relationships in metadata now will enable the evolving infrastructure to leverage this mapping.

---

Examples

A single journal article is published in two languages with each being assigned its own DOI. In this factious example both are published in the same journal. The original language instance has metadata that contains no indication of the translation instance. The alternate language instance includes in its metadata a relation to the original language instance:

<journal_article publication_type="full_text">
  <titles>
     <title>Um artigo na língua original, que passa a ser o inglês</title>
     <original_language_title language="en">An article in its original language which happens to be English</original_language_title>
   </titles>
   <contributors>
     <person_name sequence="first" contributor_role="author">
       <given_name>Daniel</given_name>
       <surname>Stepputtis</surname>
       <ORCID authenticated="true">http://orcid.org/0000-0003-4824-1631</ORCID>
     </person_name>
  </contributors>
  <publication_date media_type="online">
     <month>02</month>
     <day>28</day>
     <year>2013</year>
  </publication_date>
  <program xmlns="http://www.crossref.org/relations.xsd">
     <related_item>
       <description>Portuguese translation of an article</description>
       <intra_work_relation relationship-type="isTranslationOf" identifier-type="doi">10.5555/original_language</intra_work_relation>
     </related_item>
  </program>
  <doi_data>
     <doi>10.5555/translation</doi>
     <resource>http://www.crossref.org/</resource>
  </doi_data>
</journal_article>

---

Example

This example has a book review published as an article in the journal The Holocene. The article's title, taken from the publisher's site is "Book Review: Understanding the Earth system: compartments, processes and interactions" where this book has the DOI 10.1007/978-3-642-56843-5.

The current metadata for the review article gives no indication of the actual book being reviewed:

Modifications to the review's metadata show how it would include a relationship to the book

Meanwhile, the book's deposited metadata shows no indication of the relation to the review article

Relations API show the relationship from the point of view of both DOIs 

Book DOI's metadata showing the relationship

---

Example

An article having a Crossref DOI identifies that data represented by a DataCite DOI was used in the research and was mentioned in the article's acknowledgment section.

The  article's Crossref deposited XML:

The relationship API view of the DataCite DOI