When completing a literature review for a research paper or thesis in education, you want to make sure the majority of your articles are primary sources, preferably empirical studies. Note that including some secondary sources and monographs (books) is perfectly appropriate, but choose them carefully.
In the literature review, you want to review theory underlying your research, and review recent literature (last five to ten years) pertinent to your topic. Studies directly related to your research are preferred; however, if these do not exist, include tangentially related studies.
Primary sources are original materials, created at the time of the event or soon thereafter. They are usually created by those who saw an event or collected data themselves. So, primary, empirical studies are the first publication of the results of original research conducted by the author(s). The research can be of diverse types, such as surveys, interviews, observation, etc. The primary source MUST be published in a peer reviewed journal!!
So, when you are searching a database, such as ERIC, for primary sources, always use the "peer reviewed" limiter and limit by date. If you include such keywords as "study," "research," or "methodology" in your search string, you can bump up appropriate sources to the top of your list (if relevance ranked).
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For a complete understanding of the type of research conducted and reported upon in education, I strongly advise taking the time to read A Policy Maker's Primer on Educational Research. Do pay certain attention to "How Do I Know If The Research Is Trustworthy?".
