Reviews
Part of Essay research guide: questions, search paths, notes, and source judgment
Primary, secondary, reference, and commentary sources compared
Source types compared by proximity, purpose, strengths, limits, suitable claims, checks, and the roles of primary, secondary, reference, and commentary material.
What to take away
Primary, secondary, reference, and commentary sources compared here by proximity, purpose, and strengths. Limits, suitable claims, checks, and roles also shape the comparison.
- Source type depends on the research question, not the object's permanent label.
- Primary material gives direct access to evidence but rarely supplies complete context.
- Secondary work analyzes evidence through methods that also need evaluation.
- Reference sources orient research and lead to stronger records or scholarship.
- Commentary can reveal arguments and reception without proving the event it discusses.
Writers often rank source types as if primary always means true and commentary always means weak. A better question is: What role can this item perform for this claim?
The same newspaper article can be a primary source for the rhetoric of its time and a weak secondary source for an event it reports from rumor.
Comparison table
Source types at a glance
Primary
- Relationship
- Participant or artifact
- Strong use
- Exact record
- Main limit
- Partial or interested
- Verification
- Provenance and context
Secondary
- Relationship
- Later analysis
- Strong use
- Context and synthesis
- Main limit
- Inherits gaps
- Verification
- Method and citations
Reference
- Relationship
- Organized overview
- Strong use
- Vocabulary and leads
- Main limit
- Compressed
- Verification
- Follow citations
Commentary
- Relationship
- Reaction or advocacy
- Strong use
- Positions and framing
- Main limit
- Selective or unsupported
- Verification
- Attribute and verify
Named examples
- Primary
- Library of Congress collections, National Archives Catalog, Federal Register, U.S. Census Bureau tables
- Secondary
- Jill Lepore's These Truths, Eric Foner's Reconstruction, The American Historical Review, JSTOR
- Reference
- Oxford English Dictionary, Encyclopaedia Britannica, WorldCat, Library of Congress Subject Headings
- Commentary
- The New York Times editorial board, National Review, The Nation, C-SPAN
Proximity to subject
- Primary
- Created by a participant, institution, observer, or process under study
- Secondary
- Later analysis or interpretation of evidence
- Reference
- Organized overview or lookup work
- Commentary
- Reaction, opinion, criticism, or advocacy
Purpose
- Primary
- Direct evidence of what was created or observed
- Secondary
- Informed interpretation and context
- Reference
- Orientation and discovery
- Commentary
- Evidence of argument and reception, not the event
Suitable claims
- Primary
- "The document says X," "The dataset records Y under its definitions"
- Secondary
- "Scholar A argues B based on evidence C"
- Reference
- "The term means X in this dictionary," "This archive holds Y"
- Commentary
- "The group argued X," "Reviewer Y praised Z"
Strengths
- Primary
- Exact record, artifact, language, measurement, perspective
- Secondary
- Context, synthesis, method, explanation, debate
- Reference
- Vocabulary, dates, orientation, leads
- Commentary
- Positions, framing, public debate, reception
Limits
- Primary
- Partial, interested, decontextualized, or inaccessible
- Secondary
- Can inherit gaps or depend on contested choices
- Reference
- Compressed and rarely enough for central contested claims
- Commentary
- May summarize selectively or make unsupported claims
Checks
- Primary
- Establish provenance, context, purpose, completeness
- Secondary
- Inspect method, evidence base, citations, peer response
- Reference
- Follow citations to original or specialist sources
- Commentary
- Attribute the view and verify underlying facts elsewhere
Role
- Primary
- Direct evidence for the event or object
- Secondary
- Informed interpretation and scholarly context
- Reference
- Orientation and discovery
- Commentary
- Evidence of argument and reception, not the event
Primary sources
Primary sources include laws, minutes, letters, diaries, and interviews. They also include photographs, datasets, artifacts, original creative works, and direct observations.
The category changes with the question. A 1995 history book is secondary for an 1890 event but primary evidence for how that event was interpreted in 1995.
The Library of Congress describes primary sources as original documents and objects created at the time under study, while secondary sources retell, analyze, or interpret at some distance. Its guidance also stresses point of view, creator purpose, context, comparison, and contradiction.
For U.S. research, the National Archives Catalog, the Library of Congress's Chronicling America, and the Federal Register are common starting collections. The U.S. Census Bureau's decennial tables are primary datasets for population questions.
Use primary material to establish what a document says, what an image depicts, what a dataset records under its definitions, or how a participant remembers an event. Do not assume one item represents every participant or reveals a hidden motive.
Secondary sources
Secondary sources make analytical claims about primary material and other scholarship. They include monographs, journal articles, historical syntheses, and some institutional reports.
Named secondary works include Jill Lepore's These Truths, Eric Foner's Reconstruction, and The American Historical Review. JSTOR and Project MUSE host journal articles, while university presses publish monographs.
Evaluate:
- the research question and field;
- evidence selection;
- method and definitions;
- publication and review;
- author expertise and conflicts;
- whether later work corrected or complicated the conclusion.
Use a secondary source when the essay needs an informed interpretation, a wider pattern, scholarly context, or a method that the writer cannot reconstruct from raw evidence alone.
Reference sources
Encyclopedias, dictionaries, handbooks, catalogs, and indexes help writers learn the terrain. Timelines and bibliographies also identify key terms, people, dates, source collections, and debates.
Named reference tools include the Oxford English Dictionary, Encyclopaedia Britannica, WorldCat, and the Library of Congress Subject Headings. Each offers a different entry point: word history, broad overview, library holdings, or subject vocabulary.
Cornell University Library's research orientation distinguishes reference sources for background from books, articles, and other materials used for deeper work. That is a useful workflow distinction: reference material opens routes, then the writer follows them.
A named dictionary may be the right authority for a word's recorded meaning. A catalog record may establish archival metadata. But a short overview is rarely sufficient for a disputed causal claim.
Commentary
Commentary includes editorials, reviews, newsletters, columns, and advocacy posts. Public talks and social posts also count, but their value depends on the question.
Use commentary to show:
- what a named person or organization argues;
- how an event was framed for an audience;
- which objections circulated;
- how a work was received;
- what language shaped a controversy.
Named commentary outlets include The New York Times editorial board, National Review, The Nation, and C-SPAN. Pew Research Center reports can supply polling context, but they are not commentary by themselves.
Attribute the position. "The policy is unlawful" requires legal support if stated as your fact. "The association argued that the policy was unlawful" can be supported by the association's statement, while the legal question still needs qualified analysis.
Sources can change roles
Consider a CDC public-health poster:
- primary evidence for the agency's message and design;
- primary evidence for a printing method if the artifact is examined;
- not direct proof that the audience obeyed it;
- an illustration in a secondary history;
- an object described by a reference catalog;
- the subject of contemporary commentary.
Label the role in your notes. This prevents a source from quietly shifting from "shows what officials said" to "proves what citizens believed."
Build a mixed source set
For a researched essay, combine sources with distinct jobs:
- a primary record for the event or object;
- a secondary source for context or interpretation;
- another primary or secondary source that tests the first account;
- reference material for terminology and discovery;
- commentary only where reception or stated position matters.
Independence matters. Five articles derived from one press release remain one evidentiary line.
An example set for a Voting Rights Act essay includes the statute and the Congressional Record. A Journal of American History article, a regional newspaper, the Oxford English Dictionary, and a 1965 New York Times editorial fill other roles.
Common questions
Is an interview a primary source?
It is primary evidence for what that person said and recalled in that interview. It may not independently establish every event described.
Is Wikipedia a reference source?
It can orient a search and reveal cited leads. Verify contested or central claims through appropriate original and specialist sources.
Can commentary be authoritative?
Yes, for the commentator's own position or a field in which the writer has relevant expertise. The exact claim still determines fitness.
Does peer review make a secondary source automatically correct?
No. Review is a quality-control process, not a guarantee. Inspect methods, evidence, scope, and later criticism.
