The Crossref Curriculum

Relationships between different research objects

Creating relationships helps build a map of scholarly research objects. Expressing these relationships in the metadata enables the evolving infrastructure to build on this mapping.

These connections may consist of citations, or refer to publications which do not always exist as a single content item (its parts may be produced, curated, and published by different organizations and separate activities). Making these connections creates linked data, which is useful because it establishes associations and context.

Connections between research objects can be established through your reference lists, or by asserting a relationships type.

Crossref has also introduced other interlinking services that address specific types of relationships:

  • Components allow for the assignment of DOIs to the component parts of a publication (figures, tables, images) which may lead to their reuse.
  • Crossmark supports the connection of updates which have a material effect on the original work, for example: updates, corrections, and retractions.
  • Funding data supports identifying the organization that financially supports the research behind a specific publication.
  • Peer reviews support the host of outputs made publicly available about published scholarly content, for example: referee reports, decision letters, and author responses.

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.

The following modifications 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 relationships between items with a Crossref DOI, and other content items with a variety of identifiers.

We maintain an expansive set of relationship types to support the various content items that a research object, like a journal article, might link to. For data and software, we ask you to provide the following information:

  • identifier of the dataset/software
  • identifier type: DOI, Accession, PURL, ARK, URI, Other (additional identifier types are also accepted beyond those used for data or software, including ARXIV, ECLI, Handle, ISSN, ISBN, PMID, PMCID, and UUID)
  • relationship type: isSupplementedBy or references (use the former if it was generated as part of the research results)
  • description of dataset or software

Both Crossref and DataCite use this kind of linking. Data repositories which register their content with DataCite follow the same process and apply the same metadata tags. This means that we achieve direct data interoperability with links in the reverse direction (data and software repositories to journal articles).

You can see illustrations and examples of this schema in our data and software citation guide.

Declaring relationship types

The possible relationship types between content items can be as varied as the items themselves. We use a controlled vocabulary to define these relationships, in order to construct an orderly mapped network of content.

This is achieved by (i) 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 (ii) 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 relationships between a content item and the items in its bibliography
  2. Crossmark: explicit creation of update relations between an item and other items that materially affect it (for example, a retraction)
  3. Funding data: implicit creation of isFundedBy and hasAward relationships 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 with a Crossref DOI and an item with one of several possible identifiers.

Relationship types for associated research objects: intra-work (within a work)

Description Reciprocal relationship types
Expression isExpressionOf, hasExpression
Format isFormatOf, hasFormat
Identical isSameAs, isIdenticalTo
Manifestation isManifestationOf, hasManifestation
Manuscript isManuscriptOf, hasManuscript
Replacement isReplacedBy, Replaces
Variant isVariantFormOf, isOriginalFormOf

Relationship types for associated research objects: inter-work (between works)

Description Reciprocal relationship types
Basis isBasedOn, isBasisFor
Comment isCommentOn, hasComment
Continuation isContinuedBy, Continues
Data basis basedOnData, isDataBasisFor
Derivation isDerivedFrom, hasDerivation
Documentation isDocumentedBy, Documents
Part isPartOf, hasPart
Peer review isReviewOf, hasReview
Preprint isPreprintOf, hasPreprint
References references, isReferencedBy
Related material, such as a protocol isRelatedMaterial, hasRelatedMaterial
Reply isReplyTo, hasReply
Requirement requires, isRequiredBy
Software compilation isCompiledBy, compiles
Supplement, such as a dataset generated as part of research results isSupplementTo, isSupplementedBy
Translation isTranslationOf, hasTranslation
Version isVersionOf, hasVersion

General typed relations

This service allows for the creation of a typed relationship between an item with 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.

<xsd:attributeGroup name="relations_type.atts">
  <xsd:attribute name="identifier-type" use="required">
    <xsd:simpleType>
      <xsd:restriction base="xsd:string">
        <xsd:enumeration value="doi"/>
        <xsd:enumeration value="issn"/>
        <xsd:enumeration value="isbn"/>
        <xsd:enumeration value="uri"/>
        <xsd:enumeration value="pmid"/>
        <xsd:enumeration value="pmcid"/>
        <xsd:enumeration value="purl"/>
        <xsd:enumeration value="arxiv"/>
        <xsd:enumeration value="ark"/>
        <xsd:enumeration value="handle"/>
        <xsd:enumeration value="uuid"/>
        <xsd:enumeration value="ecli"/>
        <xsd:enumeration value="accession"/>
        <xsd:enumeration value="other"/>
      </xsd:restriction>
    </xsd:simpleType>
  </xsd:attribute>

When DOIs are used, a bidirectional relation is automatically created by Crossref when a relation is created in the deposit of one item in a pair. The DOI 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.

Example: translated article

A single journal article is published in two languages with each being assigned its own DOI. In this example, both are published in the same journal. The original language instance has metadata that contains no indication of the translation instance. The alternative language instance includes in its metadata a relation to the original language instance. Here is a screenshot of the relevant section in the code. Please refer to the code snippet below to see it in context.

Relation to the original language instance
<journal_article publication_type="full_text">
  <titles>
     <title>Um artigo na lĂ­ngua original, que passa a ser o inglĂŞs</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="https://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>https://www.crossref.org/</resource>
  </doi_data>
</journal_article>

Example: book review

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 https://doi.org/10.1007/978-3-642-56843-5.

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

<journal>
    <journal_metadata language="en">
        <full_title>The Holocene</full_title>
        <abbrev_title>The Holocene</abbrev_title>
        <issn media_type="print">0959-6836</issn>
        <issn media_type="electronic">1477-0911</issn>
    </journal_metadata>
    <journal_issue>
        <publication_date media_type="online">
            <month>07</month>
            <day>27</day>
            <year>2016</year>
        </publication_date>
        <publication_date media_type="print">
            <month>05</month>
            <year>2002</year>
        </publication_date>
        <journal_volume>
            <volume>12</volume>
        </journal_volume>
        <issue>4</issue>
    </journal_issue>
    <journal_article publication_type="full_text">
        <titles>
            <title>Book Review: Understanding the Earth system: compartments, processes and interactions</title>
        </titles>
        <contributors>
            <person_name sequence="first" contributor_role="author">
                <given_name>Ian</given_name>
                <surname>Fairchild</surname>
                <affiliation>Keele University</affiliation>
            </person_name>
        </contributors>
        <publication_date media_type="online">
            <month>07</month>
            <day>27</day>
            <year>2016</year>
        </publication_date>
        <publication_date media_type="print">
            <month>05</month>
            <year>2002</year>
        </publication_date>
        <pages>
            <first_page>505</first_page>
            <last_page>505</last_page>
        </pages>
        <publisher_item>
            <identifier id_type="doi">10.1191/0959683602hl565xx</identifier>
        </publisher_item>
        <ai:program xmlns:ai="http://www.crossref.org/AccessIndicators.xsd" name="AccessIndicators">
            <ai:license_ref applies_to="tdm">http://journals.sagepub.com/page/policies/text-and-data-mining-license</ai:license_ref>
        </ai:program>
        <doi_data>
            <doi>10.1191/0959683602hl565xx</doi>

B: Modifications to the review’s metadata show how it would include a relationship to the book

Here is a screenshot of the relevant section in the code. Please refer to the code snippet below to see it in context.

inter_work_relation relationship-type
<journal>
    <journal_metadata language="en">
        <full_title>The Holocene</full_title>
        <abbrev_title>The Holocene</abbrev_title>
        <issn media_type="print">0959-6836</issn>
        <issn media_type="electronic">1477-0911</issn>
    </journal_metadata>
    <journal_issue>
        <publication_date media_type="online">
            <month>07</month>
            <day>27</day>
            <year>2016</year>
        </publication_date>
        <publication_date media_type="print">
            <month>05</month>
            <year>2002</year>
        </publication_date>
        <journal_volume>
            <volume>12</volume>
        </journal_volume>
        <issue>4</issue>
    </journal_issue>
    <journal_article publication_type="full_text">
        <titles>
            <title>Book Review: Understanding the Earth system: compartments, processes and interactions</title>
        </titles>
        <contributors>
            <person_name sequence="first" contributor_role="author">
                <given_name>Ian</given_name>
                <surname>Fairchild</surname>
                <affiliation>Keele University</affiliation>
            </person_name>
        </contributors>
        <publication_date media_type="online">
            <month>07</month>
            <day>27</day>
            <year>2016</year>
        </publication_date>
        <publication_date media_type="print">
            <month>05</month>
            <year>2002</year>
        </publication_date>
        <pages>
            <first_page>505</first_page>
            <last_page>505</last_page>
        </pages>
        <publisher_item>
            <identifier id_type="doi">10.1191/0959683602hl565xx</identifier>
        </publisher_item>
        <program xmlns="http://www.crossref.org/relations.xsd">
            <related_item>
                <inter_work_relation relationship-type="isReviewOf" identifier-type="doi">
                    10.1007/978-3-642-56843-5
                </inter_work_relation>
            </related_item>
        </program>
        <doi_data>
            <doi>10.1191/0959683602hl565xx</doi>

C: Meanwhile, the book’s deposited metadata shows no indication of the relation to the review article:

<book book_type="other">
    <book_metadata language="en">
        <contributors>
            <person_name contributor_role="editor" sequence="first">
                <given_name>Eckart</given_name>
                <surname>Ehlers</surname>
            </person_name>
            <person_name contributor_role="editor" sequence="additional">
                <given_name>Thomas</given_name>
                <surname>Krafft</surname>
            </person_name>
        </contributors>
        <titles>
            <title>Understanding the Earth System</title>
        </titles>
        <publication_date media_type="print">
            <year>2001</year>
        </publication_date>
        <isbn media_type="print">978-3-540-67515-0</isbn>
        <isbn media_type="electronic">978-3-642-56843-5</isbn>
        <publisher>
            <publisher_name>Springer Berlin Heidelberg</publisher_name>
            <publisher_place>Berlin, Heidelberg</publisher_place>
        </publisher>
        <ai:program xmlns:ai="http://www.crossref.org/AccessIndicators.xsd" name="AccessIndicators">
            <ai:license_ref applies_to="tdm">http://www.springer.com/tdm</ai:license_ref>
        </ai:program>
        <doi_data>
            <doi>10.1007/978-3-642-56843-5</doi>

D: Relationships API shows the relationship from the point of view of both DOIs:

https://doi.crossref.org/relations/find?doi=10.3410/f.735157928.793558703

{
  "doi" : "10.3410/f.735157928.793558703",
  "relations" : [
    {
      "asClaimant" : true,
      "items" : [
        {
          "identifier" : "10.7554/eLife.42135",
          "description" : "F1000Prime recommendation of Chronology-based architecture of descending circuits that underlie the development of locomotor repertoire after birth.",
          "record-date" : "2019-04-16 12:25:48.0",
          "identifer-type" : "doi",
          "relation-type" : "isReviewOf"
        }
      ]
    },
    {
      "asClaimant" : false,
      "items" : [
      ]
    }
  ]
}

https://doi.crossref.org/relations/find?doi=10.7554/eLife.42135

{
  "doi" : "10.7554/eLife.42135",
  "relations" : [
    {
      "asClaimant" : true,
      "items" : [
      ]
    },
    {
      "asClaimant" : false,
      "items" : [
        {
          "identifier" : "10.3410/f.735157928.793558703",
          "description" : "F1000Prime recommendation of Chronology-based architecture of descending circuits that underlie the development of locomotor repertoire after birth.",
          "record-date" : "2019-04-16 12:25:48.0",
          "identifer-type" : "doi",
          "relation-type" : "isReviewOf"
        },
      ]
    }
  ]
}

E: Book DOI’s metadata showing the relationship

Here is a screenshot of the relevant section in the code. Please refer to the code snippet below to see it in context.

<query status="resolved">
     <doi type="journal_article">10.7554/eLife.42135</doi>
     <crm-item name="publisher-name" type="string">eLife Sciences Publications, Ltd</crm-item>
     <crm-item name="prefix-name" type="string">eLife Sciences Publications, Ltd.</crm-item>
     <crm-item name="member-id" type="number">4374</crm-item>
     <crm-item name="citation-id" type="number">104997326</crm-item>
     <crm-item name="journal-id" type="number">189365</crm-item>
     <crm-item name="deposit-timestamp" type="number">20190402090010</crm-item>
     <crm-item name="owner-prefix" type="string">10.7554</crm-item>
     <crm-item name="last-update" type="date">2019-04-02T09:00:31Z</crm-item>
     <crm-item name="created" type="date">2019-02-25T13:00:23Z</crm-item>
     <crm-item name="citedby-count" type="number">3</crm-item>
     <crm-item name="relation" type="doi" claim="isPreprintOf">10.1101/425587</crm-item>
     <crm-item name="relation" type="doi" claim="isReviewOf">10.3410/f.735157928.793558703</crm-item>
     <doi_record>
       <crossref xmlns="http://www.crossref.org/xschema/1.1" xsi:schemaLocation="http://www.crossref.org/xschema/1.1 http://doi.crossref.org/schemas/unixref1.1.xsd">
         <journal>
           <journal_metadata language="en">
             <full_title>eLife</full_title>

Example: linked dataset

An article with 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&rsquo;s Crossref deposited XML:

<doi_record>
<crossref xmlns="http://www.crossref.org/xschema/1.1" xsi:schemaLocation="http://www.crossref.org/xschema/1.1 http://doi.crossref.org/schemas/unixref1.1.xsd">
<journal>
<journal_metadata language="en">
<full_title>Journal of Psychoceramics</full_title>
<abbrev_title>Journal of Psychoceramics</abbrev_title>
<issn media_type="electronic">0264-3561</issn>
</journal_metadata>
<journal_issue>
<publication_date media_type="online">
<month>05</month>
<day>06</day>
<year>2012</year>
</publication_date>
<publication_date media_type="print">
<month>05</month>
<day>06</day>
<year>2012</year>
</publication_date>
<journal_volume>
<volume>5</volume>
</journal_volume>
<issue>11</issue>
</journal_issue>
<journal_article publication_type="full_text">
<titles>
<title>
Dog: A Methodology for the Development of Simulated Annealing
</title>
</titles>
<contributors>
<person_name sequence="first" contributor_role="author">
<given_name>Josiah</given_name>
<surname>Carberry</surname>
</person_name>
</contributors>
<publication_date media_type="online">
<month>05</month>
<day>06</day>
<year>2012</year>
</publication_date>
<publication_date media_type="print">
<month>05</month>
<day>06</day>
<year>2012</year>
</publication_date>
<pages>
<first_page>1</first_page>
<last_page>3</last_page>
</pages>
<crossmark>
<crossmark_policy>10.5555/crossmark_policy</crossmark_policy>
<crossmark_domains>
<crossmark_domain>
<domain>psychoceramics.labs.crossref.org</domain>
</crossmark_domain>
</crossmark_domains>
<crossmark_domain_exclusive>false</crossmark_domain_exclusive>
<updates>
<update type="correction" date="2012-05-12">10.5555/12345681</update>
</updates>
<custom_metadata/>
</crossmark>
<program xmlns="http://www.crossref.org/relations.xsd">
<related_item>
<description>Acknowledgement mention of dataset use.</description>
<inter_work_relation relationship-type="isBasedOn" identifier-type="doi">10.5284/1000389</inter_work_relation>
</related_item>
</program>
<doi_data>
<doi>10.5555/12345681</doi>
<timestamp>201601211508</timestamp>
<resource>
http://psychoceramics.labs.crossref.org/10.5555-12345681.html
</resource>
</doi_data>
</journal_article>
</journal>
</crossref>
</doi_record>
</query>
</body>

You can also see the article&rsquo;s Crossref deposited metadata in JSON.

The relationship API view of the DataCite DOI:

{
"doi": "10.5284/1000389",
"relations": [
{
"asClaimant": true,
"items": [
{
"identifier": "10.5555/12345678",
"description": null,
"record-date": "2015-06-25 09:30:44.0",
"identifer-type": "doi",
"relation-type": "isReferencedBy"
}
]
},
{
"asClaimant": false,
"items": [
{
"identifier": "10.5555/12345681",
"description": "Acknowledgement mention of dataset use.",
"record-date": "2016-01-21 15:17:52.0",
"identifer-type": "doi",
"relation-type": "isBasedOn"
}
]
}
]
}

Page owner: Laura J. Wilkinson   |   Last updated 2020-April-08