[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Epidemiology](https://durgical.com/en/epidemiology/) › [Evidence-based medicine](https://durgical.com/en/epidemiology/areas/ebm/) › Levels of evidence

# Levels of evidence

- Concept

The pyramid ranks studies by how well they avoid bias; design is a guide, but the quality of each study counts too.

Updated Nov 5, 2026

## Key points

- For therapy: systematic review of trials > randomised trial > cohort > case-control > case series > expert opinion.
- The pyramid changes with the question: for prognosis, the cohort is at the top; for diagnosis, the cross-sectional study against a reference standard.
- Randomisation balances known and unknown confounders, which is why trials sit high.
- A trial with high risk of bias, few events or inconsistent results moves down.
- Basic research and animal studies inform mechanisms but do not answer clinical questions on their own.

## What it is

Levels of evidence rank designs by their protection against bias for therapy questions: systematic reviews of randomised trials at the top, then trials, cohorts, case-control studies, case series and, at the base, expert opinion. Design alone is not enough: a poorly run trial can be worth less than a careful cohort, which is why systems such as GRADE judge the certainty of the evidence and not just the design.

## Worked example

- Data (made up): three cohorts with 40 000 people suggest a supplement reduces heart attacks: pooled RR 0.70.
- First reading: 1 − 0.70 = 0.30, that is, 30 % fewer heart attacks in those who take it.
- But those who take the supplement exercise more and smoke less: possible confounding.
- Later, a randomised trial with 8 000 people finds RR 1.02 (95 % CI 0.90 to 1.15).
- The trial CI includes 1: no difference shown; the cohort effect was confounding by lifestyle.
- Reading: the trial (higher level) corrects the cohorts; even so, its quality (losses, blinding) is judged before accepting it.

## Evidence pyramid for a therapy question

| Level | Design | Main risk |
| --- | --- | --- |
| Highest | Systematic review of randomised trials | Quality of included trials, publication bias |
| High | Randomised controlled trial | Allocation concealment, losses, lack of blinding |
| Middle | Cohort study | Confounding, loss to follow-up |
| Lower middle | Case-control | Recall bias, choice of controls |
| Low | Case series or case report | No comparison group |
| Lowest | Expert opinion | Personal experience, the expert's own biases |

## Markers

- SR
- RCT
- Cohort
- Case-control
- Expert opinion

## In practice

- For a therapy question, look first for systematic reviews of trials and guidelines built on them.
- For rare or late adverse effects, large cohorts and case-control studies may be the best evidence available.

## Common pitfalls

- Believing any trial beats any observational study: quality of conduct matters as much as design.
- Using the therapy pyramid for diagnosis or prognosis questions.

## Memory aid

Design sets the floor, quality sets the height: a study moves up or down the pyramid by how it was done.

## Test yourself

Think of the answer, then tap the question to see it.

## Which design sits at the top of the pyramid for therapy questions?

The systematic review (with meta-analysis when appropriate) of good-quality randomised controlled trials.

## Why does the randomised trial rank above the cohort?

Because randomisation spreads confounders, known and unknown, evenly between the groups.

## Why is design not enough to judge evidence?

Because a well-designed study can be poorly run (losses, no blinding, few events) and its certainty falls.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/epidemiology/levels-of-evidence/)

## Connections

## Related 6

- [The GRADE system](https://durgical.com/en/epidemiology/the-grade-system/)
- [Systematic review](https://durgical.com/en/epidemiology/systematic-review/)
- [Randomised controlled trial](https://durgical.com/en/epidemiology/randomised-controlled-trial/)
- [Cohort study](https://durgical.com/en/epidemiology/cohort-study/)
- [Confounding](https://durgical.com/en/epidemiology/confounding/)
- [Critical appraisal of a paper](https://durgical.com/en/epidemiology/critical-appraisal-of-a-paper/)

[← All of epidemiology](https://durgical.com/en/epidemiology/)

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 14: Completing ‘Summary of findings’ tables and grading the certainty of the evidence](https://www.cochrane.org/authors/handbooks-and-manuals/handbook/current/chapter-14) (Cochrane, 2026); [Chapter 1: Starting a review](https://www.cochrane.org/authors/handbooks-and-manuals/handbook/current/chapter-01) (Cochrane, 2026).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

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Source: https://durgical.com/en/epidemiology/levels-of-evidence/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/epidemiology/levels-of-evidence/