Evidence based guidelines & monographs.
Syntheses (Systematic Reviews / Meta-Analyses)
According to the Centre for Evidence-Based Medicine in Toronto: "A systematic review is a summary of the medical literature that uses explicit methods to perform a comprehensive literature search and critical appraisal of individual studies and that uses appropriate statistical techniques to combine these valid studies. Meta-analyses are often considered a type of systematic review where the quantitative data are combined and examined across all single studies included in the study."
Note: More systematic reviews and meta-analyses can also be found in the individual databases (Medline, CINAHL, etc.). Many databases offer limiters/filters to search for them.
Individual research studies published in journals (primary literature)