Skip to main content
It looks like you're using Internet Explorer 11 or older. This website works best with modern browsers such as the latest versions of Chrome, Firefox, Safari, and Edge. If you continue with this browser, you may see unexpected results.
Levels of Evidence / Types of Studies
Source: Nursetopia, March 2013.
Types of Studies/Evidence defined
As defined by the Cochrane Consumer Network, A systematic review summarises the results of available carefully designed healthcare studies (controlled trials) and provides a high level of evidence on the effectiveness of healthcare interventions. Judgments may be made about the evidence and inform recommendations for healthcare.
Source: Cochrane Consumer Network
Randomized Controlled Trial
A study design that randomly assigns participants into an experimental group or a control group. As the study is conducted, the only expected difference between the control and experimental groups in a randomized controlled trial (RCT) is the outcome variable being studied. Source: Study Design 101, GWU
A study design where one or more samples (called cohorts) are followed prospectively and subsequent status evaluations with respect to a disease or outcome are conducted to determine which initial participants exposure characteristics (risk factors) are associated with it. As the study is conducted, outcome from participants in each cohort is measured and relationships with specific characteristics determined. Source: Study Design 101, GWU
Case Control Study
A study that compares patients who have a disease or outcome of interest (cases) with patients who do not have the disease or outcome (controls), and looks back retrospectively to compare how frequently the exposure to a risk factor is present in each group to determine the relationship between the risk factor and the disease. Source: Study Design 101
An article that describes and interprets an individual case, often written in the form of a detailed story. Case reports often describe:
Unique cases that cannot be explained by known diseases or syndromes
Cases that show an important variation of a disease or condition
Cases that show unexpected events that may yield new or useful information
Cases in which one patient has two or more unexpected diseases or disorders
Case reports are considered the lowest level of evidence, but they are also the first line of evidence, because they are where new issues and ideas emerge. This is why they form the base of our pyramid. A good case report will be clear about the importance of the observation being reported. Source: Study Design 101
Expert Opinion defined by Merriam-Webster Dictionary
a belief or judgment about something given by an expert on the subject
A tutorial on pilot studies: The what, why and how
Thabane, L., Ma, J., Chu, R., Cheng, J., Ismaila, A., Rios, L. P., . . . Goldsmith, C. H. (2010). A tutorial on pilot studies: The what, why and how. BMC Medical Research Methodology, 10(1), 1-1. doi:10.1186/1471-2288-10-1
Study Design 101
Tutorial by George Washington University.
Describes different study designs.
Research Impact -- journal impact factor