The hierarchy of evidence is a framework used to organize research designs based on how well they can answer certain types of research questions while limiting sources of bias.
It is most commonly associated with evidence-based medicine and the health sciences, where study designs such as randomized controlled trials, systematic reviews, and meta-analyses are often prioritized when evaluating the effects of treatments or interventions.
However, the hierarchy should not be interpreted as a universal ranking of research quality. The most appropriate study design depends on the question being asked, and a study’s position in a hierarchy does not tell you how well that individual study was conducted.
How does the hierarchy of evidence work?
Evidence hierarchies are often illustrated as a pyramid, with study designs that are better suited to minimizing certain forms of bias placed toward the top.
The hierarchy of evidence organizes research designs into levels based on how well they are suited to answer particular types of questions and limit certain sources of bias. This structure can help researchers identify relevant evidence, critically evaluate studies, and decide which research may be most appropriate for their question.
For questions about the effects of treatments or interventions, well-conducted systematic reviews and meta-analyses of relevant studies are often placed near the top, followed by randomized controlled trials.
Observational and descriptive research may appear lower in traditional clinical evidence hierarchies, but these methods can be better suited to other research questions. For example, cohort studies can help researchers examine risk factors and outcomes over time, case-control studies are useful for investigating rare outcomes or potential risk factors, and cross-sectional studies can measure prevalence or associations within a population at a particular point in time.
Other methods may be appropriate for questions that do not fit neatly within a traditional clinical evidence hierarchy. Longitudinal or panel studies can track change over time, field studies can examine behavior in real-world settings, interview studies can explore people’s experiences and perspectives, and case reports can draw attention to rare or unusual clinical observations.
Different research questions may require different types of evidence. Even established evidence-ranking systems use different study designs depending on whether the question concerns treatment, diagnosis, prognosis, prevalence, or another topic.
Study design is not the same as study quality
A study type can tell you a lot about how research was conducted, but it does not determine whether the research itself is high quality.
When evaluating a paper, it is also important to consider factors such as:
Whether the study design is appropriate for the research question
How participants or data were selected
Sample size and representativeness
How outcomes were measured
Potential sources of bias or confounding
The study’s limitations
Whether the findings are relevant to the population or context you are interested in
Evidence assessment frameworks such as GRADE consider factors beyond study design, including risk of bias, inconsistency, imprecision, indirectness, and publication bias when evaluating confidence in a body of evidence.
The hierarchy of evidence in Consensus
Consensus includes research from many disciplines and classifies papers across a broad range of study types and research methods. Study type badges and filters in Consensus should not be interpreted as a ranking of research quality or as positions on an evidence pyramid.
Instead, they are designed to help you understand how research was conducted and find study types that are appropriate for your question.
For questions where an evidence hierarchy is relevant, such as evaluating the effectiveness of a clinical intervention, you may choose to prioritize study types such as randomized controlled trials, systematic reviews, or meta-analyses.
For other questions, a cohort study, field study, interview study, historical analysis, or another method may be more appropriate.
Using the hierarchy thoughtfully
The hierarchy of evidence can be a useful starting point, particularly in clinical and biomedical research, but it should be used alongside critical evaluation of the individual studies.
Rather than asking only, “Where does this study rank?”, consider asking:
Is this the right study design for my question?
Was the study conducted well?
How directly do the findings apply to what I want to know?
How does this study fit with the broader body of research?
The best evidence is ultimately evidence that is both methodologically sound and appropriate for the question you are trying to answer.
Understanding the hierarchy of evidence equips you with the tools to navigate research effectively. By prioritizing high-quality evidence and critically evaluating sources, you can enhance the credibility and impact of your work in academic or professional settings. This systematic approach ensures that decisions and conclusions are grounded in the best available research, fostering a culture of rigorous and informed inquiry.