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Evidence Based Medicine: Levels of Evidence

This guide is designed to assist health care professionals and students become effective and efficient users of the medical literature.

Is All Evidence Created Equal?

EBM Pyramid

The medical literature is immense, but only a small portion of it is immediately useful in answering clinical questions. The literature reports the whole spectrum of the scientific research process -- the long journey from in-vitro studies to double-blind randomized controlled trials. This has been called the "wedge of evidence" or the "pyramid of evidence." (for a visual representation see More Information)

An understanding of how various levels of evidence are reported and how this literature is organized will help the searcher retrieve the highest levels of evidence for a particular clinical question. High levels of evidence may not exist for all clinical questions because of the nature of medical problems and research and ethical limitations.

Adding METHODOLOGY terms and CLINICAL FILTERS to SUBJECT terms will result in the most efficient and optimal retrieval in terms of finding the highest level of evidence in answering clinical questions.

Study Design Concepts

Studies can be descriptive (or observational), such as :

  • case reports (a detailed description of a single case)
  • case series (descriptions of groups of patients with a disease)
  • cohort studies (defined populations which are followed in an attempt to determine distinguishing subgroup characteristics)

Or evaluative (interventional), such as a:

  • randomized/controlled clinical trials (a clinical trial involving one or more test treatments, at least one control treatment, specified outcome measures for evaluating the studied intervention, and a bias-free method for assigning patients to the test treatment)

Levels of Clinical Evidence in the Primary Literature

Type of Question

Type of Study Methodology

MEDLINE Filters

Therapy: information needed about
treatments (effectiveness, cost, etc.) 

Double-Blind
Randomized Controlled Trial 

Randomized Controlled Trial [PT]
double [TW] and blind [TW] 

Diagnosis: information needed about a
diagnostic test (sensitivity, accuracy, etc.)

Controlled Trial

Sensitivity and Specificity [MH]
Diagnosis [SH] 

Prognosis: information needed about the
course of the disease over time, expected
complications, etc. 

Cohort Studies
Case Control
Case Series 

Cohort Studies [MH]
Prognosis [MH]
Survival Analysis [MH]

Etiology/Harm: information needed about
causes of disease or contributing factors of disease

Cohort Studies

Cohort Studies [MH]
Risk [TW] 

Prevention: information needed about the
prevention of disease (immunization,
social factors, etc.)

Randomized Controlled Trial
Cohort Studies  

Randomized Controlled Trial [PT]
Cohort Studies [MH]
Prevention and Control [SH]

Quality Improvement: information needed
about clients' and health professionals'
experiences and concerns

Randomized Controlled Trial

Randomized Controlled Trial [PT]
Practice Guideline [PT]
Consensus Development Conference [PT]  

(Abbreviations: MH=Medical Subject Heading; PT=Publication Type; SH=Subheading; TW=Text Word)

Secondary Literature

The practice and teaching of Evidence-Based Medicine has outcome products which help the health care provider and consumer keep up with the medical literature and assess the evidence. This secondary literature synthesizes, filters, and evaluates the primary research literature. Dissemination and incorporation of valid clinical research findings into medical practice is the ultimate goal.

Filtered or Synthesized Information

Description/Definition

How to Find This Type of Information

Systematic Reviews

  • differ from traditional review articles in that conclusions are evidence-based rather than commentary
  • start with a clearly articulated question
  • use explicit, rigorous methods to identify, critically appraise, and synthesize relevant studies
  • appraise relevant published and unpublished evidence before combining and analyzing data
  • include description of how primary data sources are identified
  • assess individual studies for validity

Use Cochrane Collaboration

  • Cochrane Database of Systematic Reviews
  • York Database of Abstracts of Reviews of Effectiveness
  • Cochrane Controlled Trials Register
  • Cochrane Review Methodology Database

Use PubMed/MEDLINE searching

  • Review [PT] AND medline [TW]
  • (Quantitative OR Systematic OR Methodologic) AND (Review OR Overview)

Use PubMed Clinical Queries

  • "Systematic Review" is available as a limit option

Meta-Analyses

  • specific methodologic and statistical technique for combining quantitative data
  • type of systematic overview

Use Cochrane Collaboration
 

Use PubMed/MEDLINE searching

  • Meta-analysis [PT]
  • meta-anal* [TW] OR
  • metaanal* [TW]

Evidence-Based Practice Guidelines

  • gather, appraise, combine evidence systematically
  • include statements designed to assist practitioner and patient decisions
  • developed by professional groups, government agencies, local practices
  • include a structured abstract: objective, option, outcomes, evidence, values, benefits/harms/costs, recommendation, validation, sponsors

Use National Guideline Clearinghouse

Use Agency for Healthcare Research and Quality

Use US Preventive Services Task Force Recommendations

Critically Appraised Topics (CATs)

  • scan literature for clinically relevant studies
  • critically appraise the studies
  • provide commentary on strength of study and clinical significance
  • provided in a one-page format

Use EBM Reviews

Use Essential EvidencePLUS (UIC affiliate access)

  • InfoPOEMS

Use CATmaker software from CEBM

Decision Analyses/Decision Tools

  • studies that analyze decisions faced by clinicians for an individual patient, about clinical policy, or a global health care policy
  • application of explicit, quantitative methods to analyze decisions under conditions of uncertainty
  • risks and benefits of a decision are made fully explicit
  • decision tree is included

Use PubMed/MEDLINE searching

  • Decision Support Techniques [MH]
  • Cost-Benefit Analysis [MH]
  • decision analysis [TI]

Consensus Development Reports

  • assessments of emerging or critical health issues based on available research and expert opinion to identify needs for policy development and further research (description based on information from the NIH Consensus Development Program now 'retired' in light of "many other organizations that conduct [evidence] reviews")

Use PubMed/MEDLINE searching

  • "consensus statement" [TW] OR "consensus report" [TW]

 

Study Design Terminology

  • Control Group - the group that does not receive the treatment, etc.
  • Randomization - assigns participants by chance to either the treatment group or the control group.
  • Blinding - the participants and/or some of the researchers do not know the groups to which the participants have been assigned.
  • Loss to Follow-up/Intention to Treat - all of the participants should be accounted for even if they did not fully participate.
  • P Value - the probability of any observed differences having happened by chance.  The convention is that .05 is required.  
  • Power - the likelihood that a study will detect a statistically significant difference.  The convention is that a power of at least 80% is required.
  • See also Scientific Experiment Terminology from the Appraisal tab.

More Information

For a visual "evidence pyramid," see:

The 4-S Hierarchy of Evidence (Studies, Syntheses, Synopses, Systems) developed by R. Brian Haynes can be found at:

For a more detailed discussion, see "Levels of Evidence and Grades of Recommendations" at:

...and "Levels of Evidence #2" at:

Contact Us

This guide has been created by the University of Illinois at Chicago's Library of the Health Sciences at Peoria. 
Please feel free to contact us with any questions at lib-pref@uic.edu.