Types of Evidence
When thinking through scientific evidence, it’s important to understand that there are different types of resources, articles, and studies available about a topic. Levels of evidence help to rank and classify evidence in order to help you think about what’s most important to review in your research. There are a number of hierarchies of evidence designed to help readers determine how to weigh/compare sources. There has also been criticisms of evidence hierarchies, which is discussed in this “Understanding Health Research” guide.
Image from the Evidence-Based Practice guide by Duke University Medical Center Library & Archives
Meta-analyses and systematic reviews are at the very top of the pyramid because the researchers are synthesizing and appraising multiple high-quality studies using standardized methodologies designed to reduce bias. Below meta-analyses and systematic reviews are singular primary research studies ranked in order of strongest evidence at the top. The lower on the pyramid you go, the more studies you need to support your decision making because the evidence is weaker. As you move up the pyramid the study designs are more rigorous and allow for less bias.
Another view is the 6S pyramid of evidence, which focuses more on themes of where to start in terms of your research, more than the type of study. This pyramid goes in the order of system (e.g. evidence-based clinical information system), summaries (e.g. practice guideline, evidence summary), synopses of syntheses (e.g. pre-appraised systematic review); syntheses (e.g. systematic review), synopses of single studies (e.g. pre-appraised single study), and single studies (e.g. randomized controlled trial, cohort studies).
Image from McGill University Library, EBP guide
For a great summary of the levels of evidence pyramid, check out this video from the University of Alabama Birmingham Medical School: