Levels of evidence (sometimes called hierarchy of evidence) are assigned to studies based on the research design, quality of the study, and applicability to patient care. Higher levels of evidence have less risk of bias.
Level of Evidence |
Description |
Level 1 |
Evidence from a systematic review or meta-analysis of all relevant RCTs (randomized controlled trials). |
Level 2 |
Evidence from at least one well-designed RCT (e.g. large multi-site RCT). |
Level 3 |
Evidence from a single well-designed controlled trials without randomization (aka quasi-experimental studies) OR a systematic review of a complete BOE (integrative review of higher and lower evidence) OR mixed methods intervention studies
|
Level 4 |
Evidence from well-designed case-control or cohort studies |
Level 5 |
Evidence from systematic reviews of descriptive and qualitative studies (meta-synthesis) |
Level 6 |
Evidence from a single descriptive or qualitative study, EBP, EBQI and QI projects |
Level 7 |
Evidence from the opinion of authorities and/or reports of expert committees, reports from committees of experts and narrative and literature reviews |
*Adapted from: Melnyk, & Fineout-Overholt, E. (2023). Evidence-based practice in nursing & healthcare: A guide to best practice (Fifth edition.). Wolters Kluwer.
"Evidence Pyramid" is a product of Tufts University and is licensed under BY-NC-SA license 4.0
Tufts' "Evidence Pyramid" is based in part on the Oxford Centre for Evidence-Based Medicine: Levels of Evidence (2009)
Levels of Evidence (LoBiondo-Wood & Haber 2022)
Level |
Research Designs |
---|---|
1 | Systematic Review or meta-analysis of RCTs (randomized control trials) |
2 | Randomized control trials |
3 | Quasi-experimental Studies |
4 | Non-experimental studies |
5 | Meta-synthesis |
6 | Qualitative studies |
7 | Expert opinions: reports from expert panels and organizations, not based on research |
Adapted from LoBiondo-Wood, G. & Haber, J. (2022). Nursing research: Methods and critical appraisal for evidence-based practice (10th ed.). Elsevier.
Different types of clinical questions are best answered by different types of research studies. You might not always find the highest level of evidence (i.e., systematic review or meta-analysis) to answer your question. When this happens, work your way down to the next highest level of evidence.
This table suggests study designs best suited to answer each type of clinical question.
Clinical Question |
Suggested Research Design(s) |
---|---|
All Clinical Questions |
Systematic review, meta-analysis |
Therapy |
Randomized controlled trial (RCT), meta-analysis |
Etiology |
Randomized controlled trial (RCT), meta-analysis, cohort study |
Diagnosis |
Randomized controlled trial (RCT) |
Prevention |
Randomized controlled trial (RCT), meta-analysis |
Prognosis |
Cohort study |
Meaning |
Qualitative study |
Quality Improvement |
Randomized controlled trial (RCT) |
Cost |
Economic evaluation |