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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.

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

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.

          Example:

          

• Gold Standard for high-quality systematic reviews
• Abstracts for Cochrane Reviews are indexed in PubMed with links to the full-text
• Cochrane Reviews include completed reviews and protocols (reviews in progress)
• Consists of detailed, structured topic reviews of hundreds of articles. Teams of experts complete comprehensive literature reviews, evaluate the literature, and present summaries of the findings of the best studies.
• Published by the International Cochrane Collaboration.

Meta-Analysis 
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. 

          Example:

          

• PubMed Clinical Queries no longer limited to Systematic Reviews. This limit can be found with other PubMed filters.
• Identifies systematic reviews and other similar types of studies found in biomedical journals included in the PubMed database.  PubMed gathers together a much larger collection than other evidence-based practice resources
• No evaluation of comparative quality of different reviews on a topic
• Abstracts supplied by authors / journals.  No separate evaluation of quality of research
• These articles * may * require critical appraisal

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. 

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." 

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. 

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. 

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. 

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. 

     Example:

• High quality medical topic summaries related to patient care.
• Grades the available evidence.
• Part of Clinical Key
• Point -of-care tool

Editorial
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. 

Opinion
A belief or conclusion held with confidence but not substantiated by positive knowledge or proof. 

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.

In Vitro Research
In the laboratory (outside the body). The opposite of in vivo (in the body). 

 

More Definitions of Study Designs PDF