Level of evidence in research refers to a ranking system that assesses the strength and reliability of scientific research. Studies are graded based on the rigor of their design, from high-level evidence like systematic reviews and randomized controlled trials to lower-level evidence like expert opinion or case reports. This concept is sometimes called the hierarchy of evidence and often contributes to assessing the overall strength of evidence used in clinical recommendations.
The following is a standard ranking system, from highest to lowest level of evidence:
- Systematic reviews and meta-analyses of randomized controlled trials (RCTs): synthesize results from multiple high-quality RCTs to provide the strongest available evidence.
- Randomized controlled trials (RCTs): experimental studies where participants are randomly assigned to treatment or control groups.
- Cohort studies: observational studies that follow groups over time to compare outcomes based on exposure or treatment.
- Case-control studies: observational studies that compare individuals with a condition to those without, looking backward to identify possible causes.
- Cross-sectional studies: studies that examine data at a single point in time to identify patterns or associations.
- Case series and case reports: descriptive studies focusing on one or a few patients with a particular condition or treatment.
- Expert opinion and anecdotal evidence: conclusions based on clinical experience or reasoning without formal research.
For example, clinical practice guidelines typically give the most weight to systematic reviews and RCTs when recommending treatments, while lower levels of evidence, such as case reports or expert opinion, are used more cautiously, especially when higher-level evidence is unavailable.