Introducing InTRePIDs

InTRePIDs (In-Text Reference Pointer Identifiers) are new globally unique persistent identifiers for in-text reference pointers (aka in-text citations), those textual devices (e.g. “[1]”, or “(Peroni and Shotton 2012)”) that appears in the main text of a citing work, uniquely denote individual bibliographic references within that publication’s reference list, and relate to the citations between the citing and the cited publications which those bibliographic references instantiate.

InTRePIDs were developed by OpenCitations with the help of funding from the Wellcome Trust [1], were first presented at PIDapalooza 2020 [2], are described in [3], and are formally defined in [4].

InTRePIDs build upon Open Citation Identifiers, first presented at PIDapalooza 2018 [5], described in [6], and formally defined in [7], and can thus be assigned to any in-text reference pointer for which the related citation is open [as defined in 8] and has been assigned an OCI.

An InTRePID can be associated with appropriate metadata, such as the position in the document at which the relevant in-text reference pointer is located, and thus facilitate in-depth scholarship on in-text reference pointer locations and citation functions, and fine-grained analysis of the relationships between publications, by making it possible

  • to identify each in-text reference pointer with a unique PID,
  • to distinguish references that are cited only once from those that are cited multiple times,
  • to see which references are cited together (e.g. in the same sentence or within an in-text reference pointer list),
  • to determine from which section(s) of the article references are cited (e.g. Introduction, Methods, Discussion), and, potentially,
  • to determine the rhetorical function of the citations from analysis of their textual contexts, by the application of natural language processing, machine learning and artificial intelligence techniques to conduct sentiment analysis on the citation contexts.

OpenCitations maintains an InTRePID resolution service at

We hope that InTRePIDs will become a useful feature of the open science PID landscape.


[1] Wellcome Trust Open Research Fund Grant (2018). Open Biomedical Citations in Context Corpus. Awarded to Silvio Peroni.

[2] David Shotton, Marilena Daquino and Silvio Peroni (2020). In-Text Reference Pointer Identifiers – InTRePIDs. Zenodo. DOI:

[3] David Shotton (2020). Introducing InTRePIDs – In-Text Reference Pointer Identifiers. OpenCitations Blog.

[4] David Shotton, Marilena Daquino and Silvio Peroni (2020). In-Text Reference Pointer Identifier: Definition. Figshare .

[5] David Shotton (2020). Citations as First Class Data Entities, and Open CItation Identifiers. PIDapalooza 2018.

[6] David Shotton (2020). Citations as First-Class Data Entities: Open Citation Identifiers. OpenCitations Blog.

[7] Silvio Peroni and David Shotton (2019): Open Citation Identifier: Definition. Figshare.

[8] Silvio Peroni and David Shotton (2018). Open Citation: Definition. Figshare.