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Evidence-Based Medicine: Levels of Evidence

Levels of Evidence

Quality of Evidence

In evidence-based medicine, all evidence is not equal.  Study design is used to measure quality of evidence, and levels of quality are commonly represented in a pyramid.  Quality ascends the pyramid (bottom to top) with study types most susceptible to bias (low quality) at the bottom of the pyramid and study types least susceptible to bias (high quality) at the top of the pyramid.  

Learn more about study design from George Washington University's Himmelfarb Health Sciences Library.

Study Design

Definitions from the Cochrane Community Glossary 

Case Study

A study reporting observations on a single individual.

Case Series

A study reporting observations on a series of individuals, usually all receiving the same intervention, with no control group.

Case Control Study

A study that compares people with a specific disease or outcome of interest (cases) to people from the same population without that disease or outcome (controls), and which seeks to find associations between the outcome and prior exposure to particular risk factors. This design is particularly useful where the outcome is rare and past exposure can be reliably measured. Case-control studies are usually retrospective, but not always.

Cohort Study

An observational study in which a defined group of people (the cohort) is followed over time. The outcomes of people in subsets of this cohort are compared, to examine people who were exposed or not exposed (or exposed at different levels) to a particular intervention or other factor of interest. A prospective cohort study assembles participants and follows them into the future. A retrospective (or historical) cohort study identifies subjects from past records and follows them from the time of those records to the present. Because subjects are not allocated by the investigator to different interventions or other exposures, adjusted analysis is usually required to minimize the influence of other factors (confounders).

Non-Randomized Controlled Trial

Any quantitative study estimating the effectiveness of an intervention (harm or benefit) that does not use randomization to allocate units to comparison groups (including studies where ‘allocation’ occurs in the course of usual treatment decisions or peoples’ choices, i.e. studies usually called ‘observational’). To avoid ambiguity, the term should be substantiated using a description of the type of question being addressed. For example, a 'nonrandomized intervention study' is typically a comparative study of an experimental intervention against some control intervention (or no intervention) that is not a randomized controlled trial. There are many possible types of nonrandomized intervention study, including cohort studies, case-control studies, controlled before-and-after studies, interrupted-time-series studies and controlled trials that do not use appropriate randomization strategies (sometimes called quasi-randomized studies).

Randomized Controlled Trial

An experiment in which two or more interventions, possibly including a control intervention or no intervention, are compared by being randomly allocated to participants. In most trials one intervention is assigned to each individual but sometimes assignment is to defined groups of individuals (for example, in a household) or interventions are assigned within individuals (for example, in different orders or to different parts of the body).

Clinical Guideline

A systematically developed statement for practitioners and participants about appropriate health care for specific clinical circumstances.

Systematic Review

A review of a clearly formulated question that uses systematic and explicit methods to identify, select, and critically appraise relevant research, and to collect and analyze data from the studies that are included in the review. Statistical methods (meta-analysis) may or may not be used to analyze and summarized the results of the included studies.


The use of statistical techniques in a systematic review to integrate the results of included studies. Sometimes misused as a synonym for systematic reviews, where the review includes a meta-analysis.

Evidence Pyramids

Specific Representation (George Washington University)















Broad Representation (McMaster University)












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  • Study type to resource map is provided in this Guide on the Search Tips page


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