Explainer · Plain-language

What is a tertiary source?

A tertiary source indexes, compiles, or distils information from primary and secondary sources — for example encyclopaedias, dictionaries, databases, and bibliographies used to locate evidence.

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A finding aid and a digest

A tertiary source sits furthest from the original evidence: it collects, organises, or condenses what primary and secondary sources contain. Two broad kinds exist. Reference works — encyclopaedias, dictionaries, handbooks, fact books — distil established knowledge into concise summaries. Finding tools — bibliographies, indexes, and abstracting databases such as Scopus or PubMed — do not summarise content so much as point you to it. Both are designed for efficient access rather than for presenting new analysis, which is why they are described as tertiary: they package and route knowledge that originates elsewhere.

When to use one

Tertiary sources are ideal at the start of a project. Use an encyclopaedia to get oriented in an unfamiliar topic, a dictionary to pin down terminology, and an abstracting database to locate the primary studies and secondary reviews you will actually cite. They give you the lay of the land — key concepts, major figures, and where the literature lives — quickly and reliably. The standards community treats well-maintained reference and indexing infrastructure as part of the scholarly record, but as a route to evidence rather than as evidence in itself.

Why they are rarely cited as evidence

Because a tertiary source is a digest or pointer, it does not let a reader inspect the underlying data or reasoning. Citing an encyclopaedia entry for a substantive claim hides the primary research behind a summary that may simplify, age, or omit caveats. Good practice is to follow the tertiary source to the primary and secondary works it draws on and cite those instead. The exception is when the tertiary source itself is your object of study — for example analysing how a database indexes a field, where the index becomes primary evidence for that question.

Key facts

At a glance

Definition: a work that indexes, compiles or distils primary and secondary sources
Examples: encyclopaedias, dictionaries, databases, bibliographies, handbooks
Two kinds: reference digests and finding aids (indexes, abstracts)
Best use: orientation and discovering primary and secondary material
Caution: rarely cited as evidence — several steps from the data
Tip: follow it to the primary source and cite that instead

Common misconceptions

What people often get wrong

Often heard: A tertiary source is just a low-quality secondary source.

Actually: They serve a different function. Tertiary sources index or digest material for discovery and quick reference, whereas secondary sources offer original interpretation and analysis of primary evidence.

Often heard: You should cite encyclopaedias and databases directly as evidence in research.

Actually: Tertiary sources are best used to locate evidence, not as evidence. Trace the claim to the primary or secondary work behind it and cite that instead.

Often heard: Databases like PubMed or Scopus are secondary sources because they contain studies.

Actually: They are tertiary finding aids. They index and point to studies rather than reporting or interpreting findings, which makes them discovery tools rather than evidence.

Going deeper