How to use sources

Citation Styles and Sources


Over the course of your college career, you will probably come across essay writing assignments that require you to only use certain kinds of sources. Three key types of sources are:

  • 1. primary sources
  • 2. secondary sources
  • 3. peer-reviewed sources

You will also be required to use citation styles, or formal documentation and formatting rules. A few major citation styles are most commonly used (depending on the discipline). You can also download full style manuals* for each type using the links in the list below:

  1. MLA - Modern Language Association (MLA manual in PDF)
  2. APA - American Psychological Association (APA manual in PDF)
  3. Chicago Manual of Style (CMS)
  4. Turabian Style (Turabian manual in PDF)

The present guide will provide an overview of each major type of source, as well as an introduction to the different citation styles. By the end of the guide, you should be aware of both the main features of each type of source and how to identify the type to which any given source in front of you belongs.


Primary sources

A primary source is a creative or intellectual work that contains direct, firsthand knowledge of the subject or field of knowledge under consideration. The meaning of primary source changes a little depending on whether we are talking about the humanities or the sciences. In the humanities, it refers to a work produced by an artist or philosopher. In the sciences, it refers to studies that contain direct and original research.

Primary source in humanities vs. sciences





Work that has been directly produced by an artist or philosopher; firsthand accounts of historical events

Studies that contain original research directly conducted by the authors of the works


Novels, poems, philosophical treatises, diary entries, fieldwork

Scientific experiments, qualitative interviews, fieldwork

A primary source is designated as "primary" exactly because it produces new knowledge that was not present before the creation of that source. In the humanities, the new knowledge can take the form of creative inspiration or eyewitness accounts of events that have happened. In the sciences, the new knowledge can take the form of data from experiments as well as data from interviews with subjects. Original fieldwork counts as a primary source in both the humanities and the sciences.

Secondary sources

If a primary source contains original work and knowledge, then the second source is defined by the fact it primarily consists of commentary on a primary source. This can include criticism, analysis, or evaluation of a primary source. For example, a book-length commentary on the works of Dostoevsky (a Russian novelist) would generally qualify as a secondary source, since it would be a work that is commenting on a primary source (i.e. the Russian novels themselves).

Book and film reviews, popular magazine summaries of scientific studies, and the like always count as secondary sources.

Admittedly, the line between the primary source and secondary source can sometimes get a little blurry. This is because sometimes, the commentary itself can become so inspired that it seems like it should count as its own form of original knowledge. For example, the literary critic Northrop Frye has a book of commentary on the poet William Blake. Frye's book would be a secondary source since it is commentary on the primary source of Blake's poetry; however, Frye's book is also so powerful and original that it could also be treated as a primary source in its own right, depending on the context.

Frye's commentary on Blake

A primary source or a secondary source?—sometimes the line can get blurry.

In the sciences, secondary sources can also be articles that summarize findings from previous research in a given field. It is important to remember that just because an article is published in an academic magazine does not mean it is a primary source.

Here is a comparison of primary vs. secondary sources.


primary source

secondary source


Contains original research and/or knowledge

Provides commentary on a primary source


Novels, movies, original research studies

Literary criticism, movie reviews, literature reviews

Peer-reviewed sources

A peer-reviewed source can be either a primary source or a secondary source. What defines the peer-reviewed source is that it has gone through a rigorous academic process called peer-review before it was published. This means that experts in the field have confirmed that the source does, in fact, qualify as a legitimate contribution to the field.

A peer-reviewed source is generally an article published in an academic journal, such as the Journal of Advanced Nursing. There are entire databases, including Google Scholar, JSTOR, and EBSCOhost's CINAHL, that contain huge compendiums of journal articles. You can set the filters on your searches in order to ensure that only peer-reviewed articles are retrieved.

Book-length works may also qualify as peer-reviewed sources. This is usually the case if the book has been published by a university press. In addition, if the author of the book has significant credibility within his field, then this would also be a good sign that his work has been vetted over the course of his career.

Finally, it is worth noting that some websites are also peer-reviewed. The Stanford Encyclopedia of Philosophy, for example, is a peer-reviewed source even though it is an open-access website. In general, though, you should take extreme care when evaluating websites, and most websites are not peer-reviewed. If they are, then they are more like to have ".edu" at the end of their URLs.

So, here is a summary of the features of peer-reviewed sources.


a source that has gone through the academic vetting process known as peer review

Primary or secondary source?

Can be either


Articles in academic journals, books published by university presses, some very select websites

Where to find them

Academic databases, university presses


This guide has developed an overview of primary sources, secondary sources, and peer-reviewed sources for your reference. To summarize:

  • 1. A primary source contains original knowledge or research in the humanities or sciences.
  • 2. A secondary source contains commentary on a primary source.
  • 3. A peer-reviewed source has gone through a rigorous vetting process before publication.

If you have any confusion about the sources you are planning on using, then you should consider seeking advice from an expert. Resources should be available at your college library, and there are also professionals you can consult with online in order to figure out whether you are using the right kinds of sources for your college essay.

* Citation style manuals are available from third-party websites. Writer Tools does not condone copyright infringement and you are responsible for the use and reliability of third party downloads and tools. Writer Tools makes no claims or warranties.