These articles are written by the researchers who performed the actual study in the lab or in another setting. They typically include methods, results, and discussion sections related to a single research project. These articles are considered primary sources.
A detailed analysis of an individual or group as a model for a particular medical phenomena. These articles' impact is limited due to sample size, but they can serve as a starting point for more in-depth research.
An article that summarizes and synthesizes the findings of numerous original research articles. Reviews are heavily used by researchers who wish to keep up with the literature on a topic. These articles can be used to identify original research (i.e., primary sources) for further reading. Reviews are considered secondary sources.
These types of studies employ a systematic method to analyze and synthesize the results of numerous studies. Systematic reviews are widely used in the health sciences, often to find a generalized conclusion from multiple evidence-based studies (e.g., "85% of the studies showed a positive correlation with...").
A systematic method that uses statistics to analyze the data from numerous studies. The researcher will combine the actual data from studies with similar data types (i.e., males between 25-30 years old with vitamin D deficiency) and analyze them as a single expanded data set. PubMed treats meta-analysis as a subset of systematic reviews.