Purpose
Levels of evidence describe the strength of evidence provided by a study design. They help the reader judge how confidently a study can support a conclusion, but they do not replace appraisal of methodological quality.
Therapeutic studies
A commonly used hierarchy is:
Level I
High-quality randomised controlled trial or systematic review of Level I trials.
Level II
Lower-quality randomised trial, prospective comparative study or systematic review of Level II studies.
Level III
Case-control study, retrospective comparative study or systematic review of Level III studies.
Level IV
Case series without a comparison group.
Level V
Expert opinion or mechanism-based reasoning without direct clinical evidence.
Diagnostic studies
Higher levels are assigned to studies that compare an index test with an appropriate reference standard in a representative consecutive population and use blinded interpretation where possible.
Prognostic studies
Higher-quality prognostic evidence comes from prospective inception cohorts with adequate follow-up and appropriate adjustment for confounding.
Important limitations
- A level of evidence reflects study design, not automatically study quality.
- A poorly conducted randomised trial may be less reliable than a well-conducted observational study.
- Clinical relevance depends on effect size, precision, applicability, bias and consistency across studies.
Systematic reviews
A systematic review inherits the limitations of the studies it includes. Meta-analysis does not correct poor primary data.
Practical appraisal
When reading a paper, ask:
- Is the research question clearly defined?
- Was the study design appropriate?
- Were allocation and outcome assessment adequately protected from bias?
- Was follow-up sufficient?
- Are the effect size and confidence intervals clinically meaningful?
- Are the results applicable to the patient or population of interest?