In academic research, it's important to be able to distinguish between different types of sources. These differences often are contextual, meaning that a single source might fit in different categories depending on how you are using it and in what academic discipline you are writing.
Primary sources are the raw materials of scholarship.
Secondary sources report on or interpret primary sources.
Tertiary sources synthesize and present overviews of primary and secondary sources.
Scholarly sources present sophisticated, researched arguments using both primary and secondary sources and are written by experts.
Popular sources aim to inform or entertain and are intended for a general, non-specialized audience. In academic writing, popular sources most often are analyzed as primary sources.
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Research, like writing, is an incredibly creative process. As you encounter and sift through sources, you will find yourself shaping your argument in perhaps unexpected ways. The ultimate goal of research is not "to find the right answer," but rather, to create a persuasive argument based on your synthesis, analysis, and interpretation of the sources you use. For this reason, the choices you make about which sources to use as you craft your argument are of the upmost importance.