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Evidence-Based Practice: Levels of Evidence and Study Designs

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

Evidence Pyramid

An evidence pyramid visually depicts the evidential strength of different research designs. The image below is one of several available renderings of an evidence pyramid.

*  Studies with the highest internal validity, characterized by a high degree of quantitative analysis, review, analysis, and stringent scientific methodology, are at the top of the pyramid.

*  Observational research and expert opinion reside at the bottom of the pyramid.



Levels of Evidence

 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 3 or more RCTs of good quality that have similar results.

 Level II

 Evidence obtained from at least one well designed RCT (eg large multi-site RCT).

 Level III

 Evidence obtained from well-designed controlled trials without randomization (ie 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.

Which Research Designs for Which Questions?

Different types of research studies are better suited to answer different categories of clinical questions. 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 the Evidence Pyramid to the next highest level of evidence.

Therapy : Which treatment does more harm than good?

RCT > Cohort Study  >  Case Control > Case Series

Diagnosis : Which diagnostic test should I use?

Prospective, blind comparison to a gold standard, ie. A controlled trial that looks at patients with varying degrees of an illness and administers both diagnostic tests -- the test under investigation and the "gold standard" test -- to all of the patients in teh study group.

Prognosis : What is the patient's likely clinical course over time?

Cohort Study > Case Control > Case Series

Etiology / Harm : What are the causes of this disease or condition?

RCT > Cohort Study > Case Control > Case Series

Prevention : How do we reduce the chance of disease by identifying and modifying risk factors?

RCT > Cohort Study > Case Control > Case Series

Cost : Is one intervention more cost-effective than another?

Economic Analysis

Quality of Life : What will be the patient's quality of life following an intervention?

Qualitative Study

Types of Study Designs

Systematic Review 
A summary of the clinical literature. A systematic review is a critical assessment and evaluation of all research studies that address a particular clinical issue. The researchers use an organized method of locating, assembling, and evaluating a body of literature on a particular topic using a set of specific criteria. A systematic review typically includes a description of the findings of the collection of research studies. (AHRQ Glossary of Terms)

A work consisting of studies using a quantitative method of combining the results of independent studies (usually drawn from the published literature) and synthesizing summaries and conclusions which may be used to evaluate therapeutic effectiveness, plan new studies, etc. It is often an overview of clinical trials. It is usually called a meta-analysis by the author or sponsoring body and should be differentiated from reviews of literature. (PubMed)

Randomized Controlled Trial 
A controlled clinical trial that randomly (by chance) assigns participants to two or more groups. There are various methods to randomize study participants to their groups. (AHRQ Glossary of Terms)

Controlled Clinical Trial 
A type of clinical trial comparing the effectiveness of one medication or treatment with the effectiveness of another medication or treatment. In many controlled trials, the other treatment is a placebo (inactive substance) and is considered the "control." (AHRQ Glossary of Terms)

Cohort Study 
A clinical research study in which people who presently have a certain condition or receive a particular treatment are followed over time and compared with another group of people who are not affected by the condition. (AHRQ Glossary of Terms)

Case Control Study
The observational epidemiologic study of persons with the disease (or other outcome variable) of interest and a suitable control (comparison, reference) group of persons without the disease. The relationship of an attribute to the disease is examined by comparing the diseased and non-diseased with regard to how frequently the attribute is present or, if quantitative, the levels of the attribute, in each of the groups. (OCEBM Table of Evidence Glossary)

Case Series
A group or series of case reports involving patients who were given similar treatment. Reports of case series usually contain detailed information about the individual patients. This includes demographic information (for example, age, gender, ethnic origin) and information on diagnosis, treatment, response to treatment, and follow-up after treatment. (OCEBM Table of Evidence Glossary)

Case Study
An investigation of a single subject or a single unit, which could be a small number of individuals who seem to be representative of a larger group or very different from it. (Dictionary of Nursing Theory and Research, Fourth Edition)

Work consisting of a statement of the opinions, beliefs, and policy of the editor or publisher of a journal, usually on current matters of medical or scientific significance to the medical community or society at large. The editorials published by editors of journals representing the official organ of a society or organization are generally substantive. (PubMed)

A belief or conclusion held with confidence but not substantiated by positive knowledge or proof. (The Free Dictionary)

Animal Research
A laboratory experiment using animals to study the development and progression of diseases. Animal studies also test how safe and effective new treatments are before they are tested in people.(NCI Dictionary of Cancer Terms)

In Vitro Research
In the laboratory (outside the body). The opposite of in vivo (in the body). (NCI Dictionary of Cancer Terms)

Centre of Evidence Based Medicine: Study Designs

According to the Centre of Evidence Based Medicine, study designs include:

  • A defined population (P) from which groups of subjects are studied
  • Outcomes (O) that are measured

Experimental and analytic observational studies also include

  • Interventions (I) or exposures (E) that are applied to different groups of subjects


Click here: CEBM  to go to a description of each research study type, their strengths and weaknesses

> Download a PDF by Jeremy Howick about study designs


Study Type Decision Tree

Use The Study Type Decision Tree to determine a study type when authors do not specifically state it.

Image: An overview of clinical research: the lay of the land.   Grimes DA, Schulz KF.Lancet. 2002 Jan 5;359(9300):57-61. doi: 10.1016/S0140-6736(02)07283-5. PMID: 11809203.

Introduction to Study Design available with more explanation at The Center for Evidence-Based Medicine (CEBM).


Most of this page (minus the decision trees) is based on the EBP Levels of Evidence page by Bev Sedlacek at Nebraska Methodist College.  Used with her gracious blessing and permission.

Credit to Shanda Hunt for sharing the usefulness of Study Type Decision Tree.