Evidence-based medicine
- Concept
Deciding with the best available evidence, clinical expertise and the patient's values, in five steps.
What it is
Evidence-based medicine (EBM) brings together the best research evidence, clinical expertise and the patient's values and preferences. It is practised in five steps: ask, acquire, appraise, apply and assess. It does not replace clinical judgement: it informs it, and it forces us to think in absolute numbers of benefit and harm.
Worked example
- Question (made up): in people over 65 with one risk factor, does a preventive treatment versus none reduce stroke over 3 years?
- Search: a systematic review of 5 trials with 12 000 patients in total.
- Appraisal: low risk of bias; stroke at 3 years: 6 % in control and 4 % on treatment.
- ARR = 6 % − 4 % = 2 %; NNT = 1 / 0.02 = 50 patients treated for 3 years to prevent one stroke.
- Harm: major bleeding in 1 % of control and 1.5 % of treated; absolute increase 0.5 %; NNH = 1 / 0.005 = 200.
- Per 1 000 treated: 20 strokes prevented and 5 extra major bleeds.
- Apply: these numbers frame the talk with the patient, who values avoiding a stroke more than the bleeding risk.
- Assess: at 6 months, adherence and events are reviewed to see whether the decision worked in practice.
The five steps of EBM
| Step | What is done | Tool |
|---|---|---|
| 1. Ask | Turn the doubt into an answerable question | PICO |
| 2. Acquire | Find the best available evidence | Databases, guidelines, systematic reviews |
| 3. Appraise | Judge validity, results and applicability | Critical appraisal, risk of bias, GRADE |
| 4. Apply | Combine with expertise and the patient's values | Shared decision-making |
| 5. Assess | Review the outcome and one's own process | Audit, follow-up |
Markers
- EBM
- PICO
- NNT
- NNH
In practice
- Showing benefit and harm per 1 000 people helps shared decisions and avoids the misleading effect of relative percentages.
- Clinical practice guidelines and systematic reviews are shortcuts for step 2: someone has already searched and appraised the evidence.
Common pitfalls
- Reducing EBM to trials only: expertise and patient values are part of the method.
- Applying a trial result to a patient very different from the participants without thinking about applicability.
Test yourself
Think of the answer, then tap the question to see it.
What are the three components of evidence-based medicine?
The best research evidence, clinical expertise and the patient's values and preferences.
What are the five steps of EBM?
Ask, acquire the evidence, appraise it critically, apply it to the patient and assess the outcome.
A treatment lowers an event from 6 % to 4 %. What is the NNT?
ARR = 2 %; NNT = 1 / 0.02 = 50 patients treated over the study period.
Connections
Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.
Sources: Chapter 1: Starting a review (Cochrane, 2026); Chapter 14: Completing ‘Summary of findings’ tables and grading the certainty of the evidence (Cochrane, 2026).
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Representative model, for educational purposes only.