Levels of evidence (sometimes called hierarchy of evidence) are assigned to studies based on the methodological quality of their design, validity, and applicability to patient care. These decisions gives the "grade (or strength) of recommendation."
Level of evidence (LOE)
Description
Level I
Evidence from a systematic review or meta-analysis of all relevant RCTs (randomized controlled trial) or evidence-based clinical practice guidelines based on systematic reviews of RCTs or three or more RCTs of good quality that have similar results.
Level II
Evidence obtained from at least one well-designed RCT (e.g. large multi-site RCT).
Level III
Evidence obtained from well-designed controlled trials without randomization (i.e. quasi-experimental).
Level IV
Evidence from well-designed case-control or cohort studies.
Level V
Evidence from systematic reviews of descriptive and qualitative studies (meta-synthesis).
Level VI
Evidence from a single descriptive or qualitative study.
Level VII
Evidence from the opinion of authorities and/or reports of expert committees.
This level of effectiveness rating scheme is based on the following: Ackley, B. J., Swan, B. A., Ladwig, G., & Tucker, S. (2008). Evidence-based nursing care guidelines: Medical-surgical interventions. (p. 7). St. Louis, MO: Mosby Elsevier.
Research Study Designs
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 Also: cohort study, case-control study, case series
Etiology
Randomized controlled trial (RCT), meta-analysis, cohort study Also: case-control study, case series
Diagnosis
Randomized controlled trial (RCT) Also: cohort study
Prevention
Randomized controlled trial (RCT), meta-analysis Also: prospective study, cohort study, case-control study, case series
Prognosis
Cohort study Also: case-control study, case series
Meaning
Qualitative study
Quality Improvement
Randomized controlled trial (RCT) Also: qualitative study