Evidence Based Medicine

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

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Is All Evidence Created Equal?

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.


simple graphic stating "Subject Terms plus Methodological Terms and Clinical Filters equal Optimal Retrieval of High Levels of Evidence"

Study Design Concepts

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

  • Case Reports [MeSH Publication Type] - "Clinical presentations that may be followed by evaluative studies that eventually lead to a diagnosis."

  • Case Series - Descriptions of groups of patients with a disease.

  • Case-Control Studies [MeSH Term] - "Comparisons that start with the identification of persons with the disease or outcome of interest and a control (comparison, referent) group without the disease or outcome of interest. The relationship of an attribute is examined by comparing both groups with regard to the frequency or levels of outcome over time."

  • Cohort Studies [MeSH Term] - "Studies in which subsets of a defined population are identified. These groups may or may not be exposed to factors hypothesized to influence the probability of the occurrence of a particular disease or other outcome. Cohorts are defined populations which, as a whole, are followed in an attempt to determine distinguishing subgroup characteristics."

Or evaluative (interventional), such as a:

  • Clinical Trial [MeSH Publication Type] - "A work that reports on the results of a clinical study in which participants are assigned to receive one or more interventions so that researchers can evaluate the interventions on biomedical or health-related outcomes. The assignments are determined by the study protocol. Participants may receive diagnostic, therapeutic, or other types of interventions."
  • Controlled Clinical Trial [MeSH Publication Type] - "A work that reports on 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. The treatment may be drugs, devices, or procedures studied for diagnostic, therapeutic, or prophylactic effectiveness. Control measures include placebos, active medicine, no-treatment, dosage forms and regimens, historical comparisons, etc. When randomization using mathematical techniques, such as the use of a random numbers table, is employed to assign patients to test or control treatments, the trial is characterized as a RANDOMIZED CONTROLLED TRIAL."
  • Randomized Controlled Trial [MeSH Publication Type] - "A work that reports on a clinical trial that involves at least one test treatment and one control treatment, concurrent enrollment and follow-up of the test- and control-treated groups, and in which the treatments to be administered are selected by a random process, such as the use of a random-numbers table."

Studies can also be time-based:

  • Follow-up Studies [MeSH Term] - "Studies in which individuals or populations are followed to assess the outcome of exposures, procedures, or effects of a characteristic, e.g., occurrence of disease."
  • Longitudinal Studies [MeSH Term] - "Studies in which variables relating to an individual or group of individuals are assessed over a period of time."

Study Design Terminology

  • Control Group - The group that does not receive the treatment, etc.
  • Randomization - Process that assigns participants by chance to either the treatment group or the control group.
  • Blinding - Process by which the participants and/or some of the researchers do not know the groups to which the participants have been assigned. 
  • Double Blinded - When both the participants and the researchers are blinded.
  • Loss to Follow-up/Intention to Treat - Research design concept that not all of the participants will complete the research study but that they should be accounted for even though 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 on the APPRAISAL page.

Levels of Clinical Evidence in the Primary Literature

Type of Question

Type of Study Methodology


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

See a Therapy Search Example

Randomized Controlled Trial 

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

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

See a Diagnosis Search Example

Controlled Clinical Trial

Sensitivity and Specificity [MH]
Diagnosis [SH] 

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

See a Prognosis Search Example

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

See an Etiology Search Example

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.

There are several types of secondary literature:

There are also several products, or tools, that synthesize the literature. To learn more about these, visit the PRE-APPRAISED SUMMARIES page.

Systematic Reviews

Systematic Reviews are NOT the same as traditional reviews.  Systematic Reviews start with a clearly articulated question and use explicit and rigorous methods.  They include a description of how primary data sources and articles are found and selected for inclusion.  They critically appraise the found evidence by assessing their validity, reliability, and other measures of quality.  Finally, Systematic Reviews combine and analyze the results of this evidence to synthesize it into an evidence based review

Sources of Systematic Reviews include:

When searching PubMed try the

  • Systematic Review - Article Types filter

Or adding one of these to your search

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


Meta-analyses are similar to Systematic Reviews, but they include a specific methodologic and statistical technique for combining quantitative data.  They provide an evidence based overview of multiple studies with combined quantitative analysis.

Sources of Meta-analyses include:

When searching PubMed try the

  • Meta-Analysis - Article Types filter

Or adding one of these to your search

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

Evidence Based Practice Guidelines

Evidence Based Practice Guidelines gather, appraise, and combine evidence systematically.  They are typically developed by professional groups, government agencies, or expert panels. They include statements designed to assist practitioner and patient decisions.  Not all practice guidelines are evidence based so look for critical information about the methodology used or about the group that produced the guideline.

Sources of Evidence Based Practice Guidelines include:

Critically Appraised Topics

Critically Appraised Topics (CATs) scan the literature for clinically relevant studies and critically appraise them.  CATs provide commentary on the strength of the study and its clinical significance in a short, one-page format. Find out more on the APPRAISAL page.

Sources of Critically Appraised Topics (CATs) include:

Decision Analyses/Decision Tools

Decision Analyses or Decision Tools are studies that analyze decisions faced by clinicians for an individual patient, about clinical policy, or about global health care policy.  They include the application of explicit, quantitative methods to analyze decisions under conditions of uncertainty. Risks and benefits of a decision are made fully explicit and a decision tree is usually included.

Sources of Decision Analyses or Decision Tools include:

When searching PubMed try adding one of these to your search

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

Consensus Development Reports

Consensus Development Reports are assessments of emerging or critical health issues, based on available research and expert opinion, to identify needs for policy development and further research.  This definition is based on information from the NIH Consensus Development Program, which has now 'retired'.)

Similarly, "Consensus Development Conference" is a MeSH Publication Type meaning "a work that consists of summary statements representing the majority and current agreement of physicians, scientists, and other professionals meeting to reach a consensus on a selected subject."

Sources of Consensus Development Reports include:

When searching PubMed try the

  • Consensus Development Conference - Article Types filter

Or adding this to your search

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


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