Sources are considered primary, secondary, or tertiary depending on the originality of the information presented and their proximity or how close they are to the source of information. This distinction can differ between subjects and disciplines.
In the sciences, research findings may be communicated informally between researchers through email, presented at conferences (primary source), and then, possibly, published as a journal article or technical report (primary source). Once published, the information may be commented on by other researchers (secondary sources), and/or professionally indexed in a database (secondary sources). Later the information may be summarized into an encyclopedic or reference book format (tertiary sources).
Source: University of Minnesota Health Sciences Library: Primary, Secondary, and Tertiary Sources in the Health Sciences
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:
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.
Secondary Sources include:
Source: University of Washington Tacoma Library: Primary & Secondary Sources
Systematic reviews – Systematic reviews are best for answering single questions (eg, the effectiveness of tight glucose control on microvascular complications of diabetes). They are more scientifically structured than traditional reviews, being explicit about how the authors attempted to find all relevant articles, judge the scientific quality of each study, and weigh evidence from multiple studies with conflicting results. These reviews pay particular attention to including all strong research, whether or not it has been published, to avoid publication bias (positive studies are preferentially published).
Source: Evans, A. T., & Mints, G. (2019). Evidence-based medicine. UpToDate.
Meta-analysis -- Meta-analysis, which is commonly included in systematic reviews, is a statistical method that quantitatively combines the results from different studies. It can be used to provide an overall estimate of the net benefit or harm of an intervention, even when these effects may not have been apparent in the individual studies [9]. Meta-analysis can also provide an overall quantitative estimate of other parameters such as diagnostic accuracy, incidence, or prevalence.
Source: Balk, E., & Bonis, P.A. (2019). Systematic review and meta-analysis. UpToDate.