Levels of scientific evidence
Research evidence comes in various forms. Levels of hierarchies can be useful for assessing the quality of evidence. In health sciences, these are portrayed as a pyramid with levels for the different types of study design. Understanding study designs can help to judge the limitations of what can be concluded from a particular study.
The evidence pyramid visually shows the evidential strength of different types of research. All levels of the pyramid are “valid” evidence, but those at the higher levels are considered the most credible.
At the top of the pyramid is evidence including systematic reviews, meta-analyses, and critical appraisals. These studies evaluate and synthesize the literature. The top of the pyramid represents the strongest evidence. Moving down the pyramid, randomised controlled trials, cohort studies and case reports can be found.
It is important to find the highest level of evidence available, but that evidence at the top of the pyramid might not always exist for a particular research question.
Click on the different levels of the pyramid to view the definition of each type of research.
Quality of scientific evidence
The evidence pyramid visually shows the evidential strength of different types of research. All levels of the pyramid are “valid” evidence, but those at the higher levels are considered the most credible.
At the top of the pyramid is evidence including systematic reviews, meta-analyses, and critical appraisals. These studies evaluate and synthesize the literature. The top of the pyramid represents the strongest evidence.
Moving down the pyramid, randomised controlled trials, cohort studies and case reports can be found. It is important to find the highest level of evidence available, but that evidence at the top of the pyramid might not always exist exist for a particular research question.
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Meta Analysis
A meta-analysis is a method of summarising the results of the same research purpose and comprehensively evaluating its combined effect. It can be an objective systematic comprehensive qualitative and quantitative statistical analysis. It has functions that improve estimates of effect construct a general review method for omitting inadequate study conclusions and reinforce the effectiveness of statistical results to yield more comprehensive and reliable study results that are more representative of the general population.
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Systematic Reviews
A systematic review is an analysis comparing and summarising the data from different studies on a certain topic.
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Randomized controlled trials
A randomized controlled trial is a study comparing the relation cause-effect into two groups, where one receives the real treatment and the other one a placebo or no treatment. Participants are randomly assigned to either the control or intervention group, reducing the selection bias.
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Cohort studies
Cohort studies are longitudinal studies that follow research subjects over a period of time to examine outcomes across different groups.
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Case control studies
A case control study is a study comparing two groups: one with and the other without a certain disease however with similar characteristics. The aim is to try identifying the factors leading to the disease.
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Case series / case reports
The case report is a specific type of research design that reports on an aspect of the management of one or two patients. It is the first piece of research writing in the health field and represents the most basic type of study design.
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Background information / expert opinion
Background information and expert opinions are not always backed by scientific studies. They include conference proceedings, textbooks, etc.