Evidence-based medicine (EBM) is, in Sackett’s classic definition, the conscientious, explicit and judicious use of current best evidence in making decisions, integrating individual expertise with the best available external evidence and with the values and preferences of the people affected. It is a methodology framing for appraising and applying research — not clinical advice in itself.
The three pillars of EBM
EBM rests on three components that must be combined, never used in isolation:
- Best available evidence — the findings of well-conducted research, appraised for validity and relevance.
- Expertise — the judgement and experience that interpret evidence in context.
- Values and preferences — what matters to the individual or population, including acceptability and burden.
Evidence alone does not make a decision. A high-certainty finding may still be the wrong choice when it conflicts with the preferences of the people involved. EBM is the disciplined act of weighing all three.
The hierarchy of evidence
Not all study designs answer a question with equal confidence. The hierarchy of evidence — often drawn as a pyramid — ranks designs by their typical resistance to bias for questions about effectiveness. Higher tiers offer stronger protection against confounding and chance, though the hierarchy is a heuristic, not an absolute law.
| Tier (strongest first) | Design | Why it ranks there |
|---|---|---|
| 1 | Systematic reviews & meta-analyses | Synthesise all eligible studies; reduce single-study chance |
| 2 | Randomised controlled trials | Randomisation balances known and unknown confounders |
| 3 | Cohort studies | Follow groups over time; vulnerable to confounding |
| 4 | Case-control studies | Compare exposed and unexposed retrospectively; recall bias |
| 5 | Case series & reports | Describe without comparison; hypothesis-generating |
| 6 | Expert opinion | Useful where evidence is absent; lowest protection from bias |
A well-conducted systematic review of randomised trials sits at the top because it combines the internal validity of randomisation with the breadth of synthesis. Expert opinion sits at the bottom not because it is worthless — it is indispensable where evidence is thin — but because it offers the least protection against bias.
Beyond the pyramid: GRADE and certainty
The simple pyramid has a known limitation: a sloppy randomised trial can be weaker than a rigorous cohort study. Modern EBM therefore separates the design from the certainty of the evidence. The GRADE approach (Grading of Recommendations, Assessment, Development and Evaluations) rates certainty as high, moderate, low or very low. It starts from the design but can downgrade for risk of bias, inconsistency, indirectness, imprecision and publication bias — and occasionally upgrade observational evidence for a large effect or a dose-response gradient. GRADE is why a review can begin with randomised trials yet still report only low-certainty evidence.
Putting EBM into practice
The familiar EBM cycle is: ask a focused question, acquire the best evidence, appraise it critically, apply it in context, and assess the outcome. Reporting standards underpin each step — PRISMA for the reviews at the top of the hierarchy and CONSORT for the trials beneath them. For definitions of these terms see the CASRAI dictionary and our research-lifecycle coverage.
Frequently asked questions
Who coined evidence-based medicine?
The term was popularised by the group associated with David Sackett, whose definition — the conscientious, explicit and judicious use of current best evidence — remains the standard reference. EBM grew from earlier work on clinical epidemiology and critical appraisal.
Does the hierarchy mean expert opinion is useless?
No. Expert opinion is essential where higher-tier evidence does not exist and provides the judgement that interprets evidence. The hierarchy ranks designs by susceptibility to bias for effectiveness questions, not by overall value.
How does GRADE differ from the pyramid?
The pyramid ranks study designs; GRADE rates the certainty of a body of evidence for a specific question. GRADE can downgrade strong designs for limitations or upgrade observational evidence, giving a more nuanced verdict than design alone.
How does EBM relate to systematic reviews?
Systematic reviews and meta-analyses sit at the top of the hierarchy because they synthesise eligible studies into the best available answer. See our explainer on systematic reviews versus meta-analyses and our author guidance.
