Subject/Research Guides: Healthcare Research and Evidence Based Practice: Evidence Based Practice Context & Resources

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 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
Cost	Economic evaluation

Types of Resources

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.