CROCI, the Croudsourced Open Citations Index

CROCI, the Crowdsourced Open Citations Index, is a new OpenCitations Index containing citations deposited by individuals, identified by ORCiD identifiers, who have a legal right to publish them under a CC0 public domain waiver.

These citations are treated as first-class data entities, with accompanying properties including the citations timespan, modelled according to the data model described in the OpenCitations Indexes page. For a full explanation of how to provide citations to be ingested in CROCI, see our introductory blog post.

CROCI was first created and released on 3 March 2019.

Citation URLs

Each citation (i.e. an individual of the class cito:Citation) is identified by an URL structured as follows:[[OCI]].

Open Citation Identifiers

Each Open Citation Identifier [[OCI]] has a simple structure: the lower-case letters "oci" followed by a colon, followed by two numbers separated by a dash (e.g., in which the first number identifies the citing work and the second number identifies the cited work.

For citations in which the citing and cited works are identified by DOIs, which includes all the COCI citations, the OCI is created in the following manner, as explained more fully here. Each case-insensitive DOI is first normalized to lower case letters. Then, after omitting the initial doi:10. prefix, the alphanumeric string of the DOI is converted reversibly to a pure numerical string using the simple two-numeral lookup table for numerals, lower case letters and other characters presented at Finally, each converted numeral is prefixes by a 050, which indicates that CROCI is the supplier of the original metadata of the citation (as indicated at

OCIs can be resolved using the OpenCitations OCI Resolution Service.

Access to CROCI data

All the data in CROCI: