A primary source in science is a document or record that reports on a study, experiment, trial or research project. Primary sources are usually written by the person(s) who did the research, conducted the study, or ran the experiment, and include hypothesis, methodology, and results.
Primary Sources include:
Gallo, E., Stelmach, M., Frigeri, F., & Ahn, D. (2018). Determining whether a dosage-specific and individualized home exercise program with consults redues fall risks and falls in community-dwelling older adults with difficult walking: A randomized control trial. Journal of Geriatric Physical Therapy 41(3), 161-172. doi:10.1519/JPT.000000000114
Niederer, D., Vogt, L., Gonzalez-Rivera, J., Schmidt, K., & Banzer, W. (2015). Heart rate recovery and aerobic endurance capacity in cancer survivors: Interdependence and exercise-induced improvements. Supportive Care in Cancer, 23(12), 3513-3520
For medical and nursing databases, you can offer narrow to "publication type," including clinical trial, or randomized control trial, in order to locate primary source materials.
Secondary sources list, summarize, compare, and evaluate primary information and studies so as to draw conclusions on or present current state of knowledge in a discipline or subject. Sources may include a bibliography which may direct you back to the primary research reported in the article. A secondary source is a summary or description of a research study written by someone other than the study investigator(s). You can identify a secondary source by determining whether the author(s) of the article did not investigate (conduct) the research described in the article, and instead, is describing research done by others.
Secondary Sources include:
Valdivia Espino, J., Guerrero, N., Rhoads, N., Simon, N., Escaron, A., &...Martinez-Donate, A. (2015). Community-based restaurant interventions to promote healthy eating: a systematic review. Preventing Chronic Disease 12(78).