All studies are not created equal, although they all have value. This Evidence Pyramid illustrates where the various study designs sit on the hierarchy, with the highest quality/fewest resources at the top, and lowest quality, more numerous resources at the bottom.
Students 4 Best Evidence. (2014). The Evidence-Based Medicine Pyramid! Available from https://www.students4bestevidence.net/blog/2014/04/29/the-evidence-based-medicine-pyramid/
|Type of Study||
will thoroughly examine a number of valid studies on a topic and mathematically combine the results using accepted statistical methodology to report the results as if it were one large study.
focus on a clinical topic and answer a specific question. An extensive literature search is conducted to identify studies with sound methodology. The studies are reviewed, assessed for quality, and the results summarised according to the predetermined criteria of the review question.
|Randomised Controlled Trials||
are carefully planned experiments that introduce a treatment or exposure to study its effect on real patients. They include methodologies that reduce the potential for bias (randomisation and blinding) and that allow for comparison between intervention groups and control (no intervention) groups. A randomised controlled trial (RCT) is a planned experiment and can provide sound evidence of cause and effect.
identify a defined population/group and follow them forward over time (prospective), to look at the variables that might relate to the development of an outcome or look back in time (retrospective) at a group that has developed an outcome to identify the risk factors. Cohort studies compare those participants who develop the outcome of interest with either those participants who do not develop the outcome (single cohort, internal control) or a similar group that has not been exposed or treated (two cohorts, external control). Cohort studies are observational and not as reliable as randomised controlled studies, since the two groups may differ in ways other than in the variable under study.
are studies in which patients who already have a specific condition are compared with people who do not have the condition. The researcher looks back to identify factors or exposures that might be associated with the illness. They often rely on medical records and patient recall for data collection. These types of studies are often less reliable than randomised controlled trials and cohort studies because showing a statistical relationship does not mean than one factor necessarily caused the other.
|Case series/Case Reports||
consist of collections of reports on the treatment of individual patients or a report on a single patient. Because they are reports of cases and use no control groups to compare outcomes, they have little statistical validity.
consists of a recommendation or comment based on the professional experience of the writer.
Adapted from: Duke University Medical Center Library. (2014).Type of Study. Retrieved from http://guides.mclibrary.duke.edu/ebmtutorial