Levels of Evidence
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 Systematic review, meta-analysis
Questions
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 Randomized controlled trial (RCT)
Improvement Also: qualitative study
Cost Economic evaluation
Types of resources: TRIP
When searching for evidence-based information, one should select the highest level of
evidence possible--systematic reviews or meta-analyses. Systematic reviews, meta-
analyses, and critically-appraised topics/articles have all gone through an evaluation
process: they have been "filtered." Information that has not been critically appraised is
considered "unfiltered."
As you move up the pyramid, however, fewer studies are available; it's important to
recognize that high levels of evidence may not exist for your clinical question. If this is
the case, you'll need to move down the pyramid if your quest for resources at the top of
the pyramid is unsuccessful.
Meta-Analysis: A systematic review that uses quantitative methods to
summarize the results.
Systematic Review: Authors have systematically searched for, appraised, and
summarised all of the medical literature for a specific topic.
Critically Appraised Topic: Authors evaluate and synthesize multiple research
studies.
Critically Appraised Articles: Authors evaluate and synopsize individual
research studies.
Randomized Controlled Trials: Include a randomized group of patients in an
experimental group and a control group. These groups are followed up for the
variables/outcomes of interest.
Cohort Study: Identifies two groups (cohorts) of patients, one which did receive
the exposure of interest, and one which did not, and following these cohorts
forward for the outcome of interest.
Case-Control Study: Identifies patients who have the outcome of interest
(cases) and control patients without the same outcome, and looks for exposure
of interest.
Background Information/Expert Opinion: Handbooks, encyclopedias, and
textbooks often provide a good foundation or introduction and often include
generalized information about a condition. While background information
presents a convenient summary, often it takes about three years for this type of
literature to be published.
Animal Research/Lab Studies: Information begins at the bottom of the pyramid:
this is where ideas and laboratory research takes place. Ideas turn into therapies
and diagnostic tools, which then are tested with lab models and animals.
Use the TRIP database to find unfiltered and filtered information sources online.
Sources:
Greenhalgh, Trisha. How to Read a Paper: the Basics of Evidence Based
Medicine. London: BMJ, 2000.
Glover, Jan; Izzo, David; Odato, Karen & Lei Wang. EBM Pyramid. Dartmouth
University/Yale University. 2006.
Levels of Evidence
The following organizations describe levels of evidence:
Oxford Centre for Evidence-Based Medicine: Levels of Evidence
Essential Evidence Plus: Levels of Evidence
