Definitions and Explanations

primary source is an original object or document from a specific time or event under study. Primary sources include historical and legal documents, interviews, eyewitness accounts, results of experiments, survey data, observations, diaries, paintings, works of literature, ancient pieces of pottery unearthed in Iraq, and much more . In the natural and social sciences, primary sources are often empirical studies — research where an experiment was done or a direct observation was made.

secondary source is anything that’s written about a primary source, such as  an essay about a novel, a  newspaper article about AIDS research, a movie review, or subsequent thoughts on The Gettysburg Address.

tertiary source is a source that collects information from primary and/or secondary sources in one place, distilling, synthesizing or otherwise summarizing  that information into a broad overview. Examples of tertiary sources are encyclopedias, Wikipedia, and textbooks.

Can a single source ever be more than one?

Not really, but it can be confusing.  For instance, if you are writing a paper about global warming, a newspaper article that discusses new research on the topic issue is a secondary source.  But if you are writing a paper about the media’s coverage of global warming, then the newspaper article is a primary source.  What you are studying changes your relationship to the material. To further muddy the water, a secondary source may very well include primary source materials in the form of pictures, statistics, or quotes, and that might work for your teacher, but it might not.  Likewise, to ensure accuracy, it is a good practice to track down the primary source if you can just to verify it.


This chart, created by librarians at the Indiana University Bloomington, illustrates kinds of primary and secondary sources by discipline:

Discipline Primary Source Secondary Source
Archeology farming tools treatise on innovative analysis of Neolithic artifacts
Art sketch book conference proceedings on French Impressionists
History Emancipation Proclamation (1863) book on the anti-slavery struggle
Journalism interview biography of publisher Randolph Hearst
Law legislative hearing law review article on anti-terrorism legislation
Literature novel literary criticism on Desolation Angels
Music score of an opera biography of the composer Mozart
Political Science public opinion poll newspaper article on campaign finance reform
Rhetoric speech editorial comment on Martin Luther King’s “I Have a Dream” speech
Sociology voter registry Ph.D. dissertation on Hispanic voting patterns

The librarians at Ithaca College also offer a good explanation of this potentially tricky concept.

When to Use Each

I think sometimes students view primary sources as the best sources–the ones that have not been adulterated by distance, time, or misrepresentation.  While that is arguably true, there are still reasons to be cautious.  For instance, even a primary source is subject to the author’s bias.  Additionally, some primary sources, such as scientific experiments,  are written for a very small audience of experts in the field. Readers won’t benefit from such sources unless they have the requisite knowledge. Still other primary sources, such as historical documents, are limited in their scope. Though they may give you a good feel for what people were thinking at the time, they cannot tell you what happened later.

Secondary sources are a good way to balance out the information gleaned from primary sources.  They can summarize, explain, and contextualize the information from a primary source, making it easier for non-experts to understand.  However, secondary sources are also subject to the bias of their authors, and sometimes egregiously so.   Unethical practitioners may lean on the inherent credibility of primary sources in order to spread misinformation. You’ve probably received e-mail forwards that say things like, “A new research study by Harvard proves _________.”  But if you took the time to read the actual study, you found that the email forward either misrepresented or misunderstood the actual study. Thus, it is a good idea to check the primary source to make sure it is being fairly represented.

But finding the primary source has in some ways become trickier in the age of the internet, now that people can video something, edit it, post it online (or on the news), and build a following and a narrative before the truth ever comes out. Consider this and this.  In the cases of Ed Schultz’s misrepresentation of George W. Bush and James O’Keefe’s misrepresentation of ACORN, what were taken as the primary sources were actually secondary sources–edited versions of the raw footage, so caveat emptor!

Tertiary sources compile and synthesize information from both primary and secondary sources to provide a summary or to advance an argument of a given subject.  Encyclopedias, textbooks, and doctoral dissertations are examples of tertiary sources.  Though these sources often provide only a general overview of a subject, a distanced summary of a field that reflects all of its ups and downs and points and counterpoints is will often provide a better overall understanding of a field than reading a single experiment or research paper.

For instance, if you read a primary source, such as Andrew Wakefield’s Ileal-lymphoid-nodular hyperplasia, non-specific colitis, and pervasive developmental disorder in children, published in Lancet, 1998, you might find it persuasive in its indication that vaccines are linked to autism. However, reading a broader overview of the field, for instance a tertiary source such as a medical textbook or a secondary source such as Good investigative reporting may finally debunk the myth that vaccines cause autism, published in Harvard Health Publications,  would reveal to you that the study was ultimately retracted by Lancet in 2010 because the research methods were flawed.

Being informed means more than finding the exact article that backs up your thesis; it means understanding how all the articles fit together within the broader structure and knowledge of the discipline. Information literacy demands attention to different kinds of soruces as well as the critical thinking necessary to evaluate each for timeliness, credibility, and bias.