Primary sources are published works generated on the front lines of science. They record new knowledge coming from field work and laboratories of the discipline. They are the source of original data which either supports or invalidates the researcher's hypothesis. They include: published reports of original research, experiments, direct observations in the field and the theories that arise from them. Examples include original research studies written up by researchers and published in scholarly journals, laboratory notebooks and other original recorded data, conference presentations on original discoveries, doctoral dissertations based on original research.
Secondary sources are published works which interpret, comment on or evaluate primary sources. Examples include published meta-analysis, systematic review articles, expert opinion, committee reports, background articles, book reviews, editorials, readers' letters to the editor, etc. Scientists who interpret, comment on or evaluate the research of other scientists have created a secondary source.
Rule of Thumb
If the author is NOT the person who did the research, it's secondary.