Types of Studies & Levels of Evidence
There are many different types of studies you will be reading about in the literature. Below is information to help you understand them better.
Levels of Evidence
A nice way to visualize both the quality of the evidence (studies) as well as the amount of it available, is this pyramid which indicates the levels of quality. The higher on the pyramid, the higher the quality of the evidence, however, there is less of it available.
Types of Studies (and their definitions)
A subset of systematic reviews; a method for systematically combining pertinent qualitative and quantitative study data from several selected studies to develop a single conclusion that has greater statistical power. This conclusion is statistically stronger than the analysis of any single study, due to increased numbers of subjects, greater diversity among subjects, or accumulated effects and results. 1
A document often written by a panel that provides a comprehensive review of all relevant studies on a particular clinical or health-related topic/question. The systematic review is created after reviewing and combining all the information from both published and unpublished studies (focusing on clinical trials of similar treatments) and then summarizing the findings. 1
A statement produced by a panel of experts that outlines current best practice to inform health care professionals and patients in making clinical decisions. The statement is produced after an extensive review of the literature and is typically created by professional associations, government agencies, and/or public or private organizations. 1
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 1
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 1
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. 1
Case control studies are observational because no intervention is attempted and no attempt is made to alter the course of the disease. The goal is to retrospectively determine the exposure to the risk factor of interest from each of the two groups of individuals: cases and controls. These studies are designed to estimate odds.
Case control studies are also known as "retrospective studies" and "case-referent studies." 1
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.
If multiple case reports show something similar, the next step might be a case-control study to determine if there is a relationship between the relevant variables. 1
Recommendations from persons with established expertise in a specific clinical area often based on clinical experience; not considered a research method because systematic (or critical) inquiry is lacking. 2
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