The Evidence Pyramid

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




  • If you start at the base of the pyramid - the first two indicate basic research
  • Next you have two types of publications that are "Level 3 Evidence"
  • Then Case Series, Case Control and Cohort Studies are Level 2 Evidence
  • Randomized Controlled Double Blind Studies are Level 1 Evidence
The "levels" are assigned these grades broadly based on the criteria of 
  • Number of patients involved in studies
  • Possibility of bias in each. The higher the possibility of bias, the lower the level of Evidence
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

Subpages (1): Sources of Evidence