What does primary vs. secondary vs. tertiary mean? 

The distinction between primary, secondary and tertiary sources hinges on how far from the original event or phenomenon the information source is created. Is it first-hand knowledge? A second-hand interpretation? A third-hand synthesis and summary of what is known? 

• Primary sources are created as close to the original event or phenomenon as it is possible to be. For example, a photograph or video of an event is a primary source. Data from an experiment is a primary source. 
• Secondary sources are one step removed from that. Secondary sources are based on or about the primary sources. For example, articles and books in which authors interpret data from another research team's experiment or archival footage of an event are usually considered secondary sources.
• Tertiary sources are one further step removed from that. Tertiary sources summarize or synthesize the research in secondary sources. For example, textbooks and reference books are tertiary sources.

Why is this important? Because different kinds of research call for using primary, secondary, and tertiary sources in different ways. For example, a research paper usually requires a combination of primary and secondary sources.

Primary, Secondary, and Tertiary Sources

The video below describes in greater detail more about the differences between primary, secondary and tertiary sources, and how each is used in research.

Examples of Primary Sources

Where To Find Primary Sources

Unlike articles and books, primary sources are often unpublished, and that can make them harder to find. But there are many tools to help you locate them.

• • The Web 
    • Use a search engine to search for your topic, adding phrases like "primary sources," "letters," "newsreel," etc.
  • Primary source databases 
    • Many libraries subscribe to databases of primary sources. The Library's primary source databases can be found in the Primary Sources subject guide.
  • Newspapers 
    • Newspapers are key primary sources for past current events. Try searching newspaper websites like the Library of Congress's Chronicling America: Historic American Newspapers.
  • Scientific research articles 
    • In the sciences, peer-reviewed research articles are considered primary sources. This makes sense, because they are full of direct evidence in the form of data. 
  • Project Gutenberg 
    • If your topic is literature, then old literature counts as a primary source. Project Gutenberg has digitized an incredible amount of public domain literature. 
  • Museums, archives, historical societies, civic and cultural groups, religious institutions, and public libraries
    • Cultural institutions often collect pictures, letters, diaries, archival materials, ephemera, etc. As it becomes faster and cheaper to do so, they are preserving this material and providing public access to it by scanning it and making it available on the Web. You will likely gain access to even more resources if you are able to visit the institution housing it in person. For starters, try Library of Congress | American Memory.
  • Government agencies and non-governmental organizations (NGO) 
    • If you need statistics and datasets, try UNdata or USA.gov | Reference Center | Data and Statistics

Primary Sources and Gray Literature

You may have noticed that there is an overlap between the topic of gray literature and the topic of primary sources. Long story short - Not all Gray Literature is Primary Sources, and not every Primary Source is an example of Gray Literature. But here are some examples of things that are both:

• Annual reports
• Government reports
• Unpublished manuscripts
• Data sets
• A researcher's notes or logs.

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