9.2.2 “Hierarchy of Evidence”
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Describes the “Hierarchy of Evidence” as it Applies to Different Types of Questions
In healthcare, the hierarchy of evidence refers to the ranking of different types of studies based on their level of evidence. The hierarchy of evidence can be used to determine the strength of the evidence in support of a particular intervention or treatment. In this teaching piece, we’ll explore the hierarchy of evidence and how it applies to different types of clinical questions.
The hierarchy of evidence can be visualised as a pyramid, with the highest quality evidence at the top and the lowest quality evidence at the bottom. At the top of the pyramid are systematic reviews and meta-analyses, followed by randomized controlled trials, cohort studies, case-control studies, cross-sectional studies, case reports, and expert opinion at the bottom.
| Level of Evidence | Positives | Negatives |
|---|---|---|
| Expert Opinion | Provides insights based on clinical experience Quick to formulate Flexible | Highly subjective Potential for bias Not evidence-based |
| Case Reports and Case Series | Can highlight rare or new conditions Generates hypotheses Provides detailed info | No control group Limited generalisability Cannot establish causality |
| Cross-sectional Studies | Snapshot of data at a specific time Useful for prevalence studies Can be less costly and time-consuming | Cannot establish causality Limited temporal data Prone to selection bias |
| Case-control Studies | Good for studying rare conditions Less time and resource-intensive than cohort studies | Retrospective nature can lead to recall bias Difficult to establish temporal relationship |
| Cohort Studies | Can observe effect over time Good for studying risk factors | Expensive and time-consuming Potential for loss to follow-up |
| Randomised Controlled Trials (RCTs) | Gold standard for testing interventions Minimises bias through randomisation | Expensive and complex Ethical considerations May lack external validity |
| Systematic Reviews and Meta-analyses | Comprehensive overview of existing research Can provide strong evidence through data aggregation | Quality depends on the included studies Potential for publication bias |
The hierarchy of evidence is particularly important when formulating different types of clinical questions, as different types of questions require different levels of evidence. Here’s how the hierarchy of evidence applies to different types of clinical questions:
Therapy Questions:
Therapy questions ask whether an intervention or treatment is effective in treating a particular condition. The highest level of evidence for therapy questions is a systematic review or meta-analysis of randomized controlled trials. If a systematic review or meta-analysis is not available, the next best option is a well-designed and conducted randomized controlled trial.
Harm Questions:
Harm questions ask whether an intervention or treatment causes harm or adverse effects. The highest level of evidence for harm questions is a systematic review or meta-analysis of randomized controlled trials or cohort studies. If a systematic review or meta-analysis is not available, the next best option is a well-designed and conducted cohort study.
Aetiology Questions:
Aetiology questions ask about the causes of a particular condition or disease. The highest level of evidence for aetiology questions is a systematic review or meta-analysis of cohort studies. If a systematic review or meta-analysis is not available, the next best option is a well-designed and conducted cohort study.
Prognosis Questions:
Prognosis questions ask about the likely course or outcome of a particular condition or disease. The highest level of evidence for prognosis questions is a systematic review or meta-analysis of cohort studies that follow a group of individuals over time. If a systematic review or meta-analysis is not available, the next best option is a well-designed and conducted cohort study.
Diagnosis Questions:
Diagnosis questions ask about the accuracy of a particular diagnostic test or procedure. The highest level of evidence for diagnosis questions is a systematic review or meta-analysis of well-designed and conducted cross-sectional studies that compare the diagnostic test or procedure to a gold standard. If a systematic review or meta-analysis is not available, the next best option is a well-designed and conducted cross-sectional study.
Economic Questions:
Economic questions ask about the cost-effectiveness of an intervention or treatment. The highest level of evidence for economic questions is a systematic review or meta-analysis of economic evaluations, which can include cost-effectiveness analyses or cost-utility analyses. If a systematic review or meta-analysis is not available, the next best option is a well-designed and conducted economic evaluation.
Qualitative Questions:
Qualitative questions ask about the experiences, beliefs, and attitudes of individuals related to a particular condition or treatment. The highest level of evidence for qualitative questions is a systematic review or meta-synthesis of qualitative studies. If a systematic review or meta-synthesis is not available, the next best option is a well-conducted qualitative study.
| Type of Question | Highest Level of Evidence |
| Therapy | Systematic review or meta-analysis of randomized controlled trials |
| Harm | Systematic review or meta-analysis of randomized controlled trials or cohort studies |
| Aetiology | Systematic review or meta-analysis of cohort studies |
| Prognosis | Systematic review or meta-analysis of cohort studies |
| Diagnosis | Systematic review or meta-analysis of well-designed and conducted cross-sectional studies |
| Economic | Systematic review or meta-analysis of economic evaluations |
| Qualitative | Systematic review or meta-synthesis of qualitative studies |
In conclusion, the hierarchy of evidence is an important tool for evaluating the strength of evidence in support of different types of clinical questions. Healthcare professionals should consider the hierarchy of evidence when formulating clinical questions and selecting the most appropriate sources of evidence to answer those questions.
References:
- Guyatt, G. H., Oxman, A. D., Vist, G. E., Kunz, R., Falck-Ytter, Y., Alonso-Coello, P., … & Jaeschke, R. (2008). GRADE: an emerging consensus on rating quality of evidence and strength of recommendations. Bmj, 336(7650), 924-926. doi: 10.1136/bmj.39489.470347.AD