Primary sources are sources that present new and original material. In the sciences, new and original research is the common thread. Information gathered from a study or experiment can be shared in many ways. Dissertations, lab notes, interviews, papers presented at professional meetings and technical reports are a few examples. Articles published in peer-reviewed journals, however, are going to be the primary sources that you will find the most helpful and easily accessed.
Secondary sources are simply everything else. Most commonly, they are sources that refer to and summarize primary sources, but they also include sources that present information that is generally recognized as well established and no longer needs supporting evidence. Examples are reviews, books, encyclopedias and handbooks. They are great for getting an overview of a topic or background information.
Read the abstract. Most will identify what type of article it is. They will use phrases like "this study will examine..." or may even say what type of study they used. Types include Randomized Controlled Clinical Trials, Controlled Clinical Trial, Case Study and others. Look for detailed METHODS, RESULTS and CONCLUSIONS/DISCUSSION sections.
Original Research Articles
Dissertations and Theses
Conference Papers and Proceedings
Case Studies and Reports