What is a Persistent Identifier (PID)?
A PID is a long-lasting digital reference to an object, contributor, or organization, “a code which remains constant as a means of identifying a digital object regardless of changes to its location on the internet” . An “identifier” is “an association between a string (a sequence of characters) and an information resource” . Web URLs are an example of a common identifier. The term "persistent" refers to the need for an identifier to provide continued access to and provenance for the object it refers to for years to come.
The long-term persistence of identifiers for objects, contributors, and organizations is vital to robust data management strategies. Publishers, funders, and other organizations have implemented PIDs in their established research workflows to enable the creation of trusted digital connections between objects, contributors, and organizations.
PID is a new name for a concept that has been a part of publishing for decades. In the past publishers used identifiers such as ISBNs and ISSNs to distinguish unique textual objects . However, the proliferation of digitally available research and technical publications has created a need for machine-readable, interoperable PIDs. Machine-readable PIDs such as DOIs and ORCID iDs are valuable assets in enabling information sharing across systems.
ISNI (ISO 27729) “is the ISO certified global standard number for identifying the millions of contributors to creative works and those active in their distribution, including researchers, inventors, writers, artists, visual creators, performers, producers, publishers, aggregators, and more” (ISNI-IA, 2019) and serves to disambiguate contributor names to improve search and discovery. The mission of ISNI is “to resolve the problem of name ambiguity in search and discovery.” ISNI identifiers and ORCID iDs are interoperable. ISNI is a global standard governed by the international standards organization (ISO) while ORCID iDs is an oepn registry where researchers can edit their own identifier page.
An ORCID iD is a PID for researchers that records professional activities and disambiguates one researcher from another. An ORCID profile connects researchers with their contributions and affiliations over time, despite name changes or different name formats (e.g. John Q. Smith, J. Smith, John Smith, JQ Smith, etc.). It can be connected in some way to most other creator profiles, and is the most interoperable creator PID. ORCID iDs can be created and edited by researchers, whereas the other identifiers listed here are created automatically or by a governing body. See the ORCID iD section of this LibGuide for more information.
ResearcherID is a unique identifier that connects researchers with their works in the Web of Science publishing ecosystem. ResearcherID profiles can be connected with ORCID iD profiles.
Scopus Author IDs are automatically assigned to authors with works indexed in Elsevier’s Scopus abstract and citation database. Scopus Author IDs can be linked with ORCID iDs.
An ARK identifier is a “specially constructed, globally unique, actionable URL and the ARK scheme is underpinned by three requirements based on links: from the object to a promise for stewardship, from the object to metadata which describes it, and to the object itself” .
A DOI is a “digital identifier of an object,” , not an identifier of a digital object—that is, DOIs can be assigned to any object, whether physical or digital. DOIs serve as “unique, permanent numbers assigned to specific objects, which remain unchanged” . DOIs are the most common type of identifier for digital objects, particularly for scholarly, research, and technical publications. See the DOI section of this LibGuide for more information.
The Handle or HNDL system is a “distributed information system designed to provide an efficient, extensible, and secured global name service for use on networks such as the Internet” . Handle serves primarily as an underlying architecture for identifier systems such as the DOI system, but it can also be used as an identifier system on its own.
The PURL system provides “a naming and resolution service for general internet resources” . A PURL is a URL that “points to a resolution service instead of the actual location of a digital resource,” and the resolution service then redirects to the current URL of the resource . ARKs, DOIs, and HNDLs are all forms of PURLs, long-lasting URLs that point to resolution services that maintain information about the current location of a resource.
Funder IDs are listed in the Crossref Funder Registry, “an open and unique registry of persistent identifiers for grant-giving organizations around the world" . Funder IDs improve transparency of research funding by linking research with grant and funder information.
The GRID collects “unambiguous institutional information” and assigns a PID and metadata including aliases and addresses to each listed research institution. The GRID is available for download and reuse under a CC0 1.0 license.
ROR IDs are “globally unique, stable, discoverable, and resolvable”  identifiers for research organizations. ROR IDs store metadata about organizations “such as alternate names/abbreviations, external URLs, and other identifiers, such as Wikidata, ISNI, and the Open Funder Registry” to support interoperability . The ROR ID database is based on seed data from GRID.
A number of commonly used researcher profile services and object and organization naming systems exist which are not considered to be PIDs. They are included here for reference because they are often brought up in relation to the identifiers listed above.
This service allows authors to track citations of their articles via a profile linked to a Google account. The profile may be public or private, and users have the option to update their profile with new publications manually, automatically, or automatically with manual review.
ResearchGate is a professional networking service for researchers where users can share publications, track citations, and connect with other users. A ResearchGate profile may contain similar info to the other creator identifiers listed here, but has no claim to persistence or permanence. ORCID iDs can be displayed on ResearchGate profiles, but there is no simple mechanism for updating publications between the two profiles.
The NAICS is “the standard used by Federal statistical agencies in classifying business establishments for the purpose of collecting, analyzing, and publishing statistical data related to the U.S. business economy.” It was developed jointly across U.S. Canadian, and Mexican statistical organizations “to allow for a high level of comparability in business statistics among the North American countries."
Wikidata is a Wikimedia Foundation repository that serves as “a free, collaborative, multilingual secondary database” that provides structured data supporting Wikipedia and other Wikimedia projects . The repository is primarily composed of items containing labels, aliases, and descriptive statements. Each item is assigned a unique identifier that is used throughout the Wikimedia ecosystem and beyond.