Hierarchy of evidence

Evidence hierarchies reflect the relative authority of various types of biomedical research. Although there is no single, universally-accepted hierarchy of evidence, there is broad agreement on the relative strength of the principal types of research. Randomized controlled trials (RCTs) rank above observational studies, while expert opinion and anecdaotal experience are ranked at the bottom. Some evidence hierarchies place systematic review and meta analysis above RCTs, since these often combine data from multiple RCTs, and possibly from other study types as well. Evidence hierarchies are integral to evidence-based medicine.

The use of evidence hierarchies has been criticized as allowing RCTs too much authority. Not all research questions can be answered through RCTs, either because of practical issues or because of ethical issues. Moreover, even when evidence is available from high-quality RCTs, evidence from other study types may still be relevant.

Ranking:

1. Systematic Reviews and Meta-Analysis

2. Randomized Controlled Trials with definite results

3. Randomized Controlled Trials with no definite results

4. Cohort Studies

5. Case-Control Studies

6. Cross-Sectional Studies

7. Case Reports

8. Expert Opinion

9. Anecdotes