As research has moved from predominantly paper based to predominantly digital, the ways in which outputs are found, cited and archived have changed. Internet searches and persistent identifiers have replaced the requirement for meticulous referencing via journal volumes, issues and pagination.

Types of persistent identifiers

A persistent identifier is a permanent, unique reference to a digital resource. There are a variety of persistent identifiers such as Digital Object Identifiers (DOIs), International Standard Book Numbers (ISBNs), International Standard Serial Numbers (ISSNs), handles, Archival Resource Keys (ARKs) and Persistent Uniform Resource Locators (PURLs). Each of these covers a different type of digital resource, with the Digital Object Identifier being most commonly used to identify individual articles and various other research objects (e.g. research data).

Other types of persistent identifiers can also be helpful:

• ORCID is used to uniquely identify authors and overcome challenges such as author ambiguity from duplicate names, name changes through marriage or authors moving institutions.
• ROR is used for identifying institutions and organisations.
• Grant DOIs are used to track the outputs connected to the research supported by a grant
• Research activity identifiers (RAiD) are used to identify research projects, their members and tools being used.  

The role of Digital Object Identifiers

The Digital Object Identifier is a system developed by the International DOI Foundation for identifying and linking to digital content. Digital Object Identifiers are used by organisations such as publishers and libraries to enable persistent access to digital content, overcoming the challenges associated with links which can change or become broken over time. The development and usage of Digital Object Identifiers has made it easier to access and cite digital research as well as to preserve articles and track citations.

There are a number of different Digital Object Identifier registration agencies. Registration agencies facilitate Digital Object Identifier registration, metadata deposit and citation information. The registration agencies also ensure that publishers adhere to industry standards (e.g. digital preservation) and that their Digital Object Identifiers continue to resolve properly. Most agencies charge a membership fee per year and then charge for each Digital Object Identifier deposited. Examples of registration agencies include:

• Crossref, a non-profit membership agency
• DataCite, a non-profit agency providing Digital Object I registry for research data
• mEDRA, is the multilingual European Registration Agency
• ISTIC, is a Chinese DOI registration agency
• Japan Link Center, is a Japanese DOI registration agency

How are DOIs structured?

Digital Object Identifiers are made up of two sections, a prefix and a suffix, which are separated by a forward slash. They are case-insensitive and can be made up of any Unicode character, though agencies may limit which character they allow. The prefix identifies an organisation, in our use case usually a publisher, and the suffix identifies the article. Different Digital Object Identifiers registration agencies mandate how the suffix should be formatted. By clicking on a DOI link, users are redirected to a chosen URL on the publisher’s website. Although this URL may vary in time, the DOI is expected to retain its connection to the target content (i.e. the article).