Publications in Type II (current and old research) and Type III resources (analyses and summaries) have a "hierarchy". This hierarchy is explained well through the Evidence Pyramid
The "levels" are assigned these grades broadly based on the criteria of
Systematic Reviews and Meta-analyses are not "Primary research" on patients as all the above mentioned studies are. These are "Research papers about primary research". They analyze the Level 1 (RCTs) and occasionally Cohort Studies and generate a Review that indicates the status of evidence based on statistics and the "risk of bias"
Note: You can check out more about each type of study through several good websites or some of the Additional Reading resources we indicate at the end of this program