Level of Evidence

Version Oxford 2011

The Oxford Centre for Evidence-Based Medicine (OCEBM) 2011 Levels of Evidence is a framework used to rate the quality and strength of evidence in medical research and clinical practice. This system was created by the Oxford Centre for Evidence-Based Medicine to guide clinicians and researchers in assessing the reliability of evidence when making medical decisions. The 2011 version offers a granular approach, categorizing levels based on the type of clinical question (e.g., therapy vs. diagnosis), which makes it useful for selecting studies specific to various types of medical queries.

Clinicians use the OCEBM Levels of Evidence to assess the quality of studies when reviewing medical literature. Higher-level evidence (Levels 1 and 2) is generally prioritized in clinical decision-making, whereas lower-level evidence (Levels 4 and 5) is relied upon when stronger studies are unavailable. This system helps clinicians make evidence-based choices, supporting patient outcomes through research-backed practices.

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Questions	Step 1 (Level 1*)	Step 2 (Level 2*)	Step 3 (Level 3*)	Step 4 (Level 4*)	Step 5 (Level 5)
How common is the problem?	Local and current random sample surveys (or censuses)	Systematic review of surveys that allow matching to local circumstances ii	Local non-random sample ii	Case-series ii	n/a
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i Level may be graded down on the basis of study quality, imprecision, indirectness (study PICO does not match questions PICO), because of inconsistency between studies, or because the absolute effect size is very small; Level may be graded up if there is a large or very large effect size.

ii As always, a systematic review is generally better than an individual study.

Reference: http://www.cebm.net/index.aspx?o=5653

OCEBM Table of Evidence Working Group = Jeremy Howick, Iain Chalmers (James Lind Library), Paul Glasziou, Trish Greenhalgh, Carl Heneghan, Alessandro Liberati, Ivan Moschetti, Bob Phillips, Hazel Thornton, Olive Goddard and Mary Hodgkinson

Version Oxford 2008

The Oxford Centre for Evidence-Based Medicine (OCEBM) 2008 Levels of Evidence is an earlier framework designed to categorize the quality of medical evidence based on study design and reliability. Like the 2011 update, it was created to help clinicians assess the strength of evidence when making healthcare decisions. The 2008 version ranks evidence according to study type.

The 2008 levels are organized from Level 1 (highest quality) to Level 5 (lowest quality).

Level Therapy / Prevention, Aetiology / Harm

1a SR (with homogeneity) of RCTs

1b Individual RCT (with "narrow Confidence Interval”)

1c All or none

2a SR (with homogeneity) of cohort studies

2b Individual cohort study (including low quality RCT; e.g., <80% follow-up)

2c “Outcomes” Research; Ecological studies

3a SR (with homogeneity) of case-control studies

3b Individual Case-Control Study

4 Case-series (and poor quality cohort and case-control studies)

5 Expert opinion without explicit critical appraisal, or based on physiology, bench research or “first principles”

Reference: Produced by Bob Phillips, Chris Ball, Dave Sackett, Doug Badenoch, Sharon Straus, Brian Haynes, Martin Dawes since November 1998. Updated by Jeremy Howick March 2009.

Cochrane RoB Tool 2.0.

The Cochrane Risk of Bias Tool 2.0 (RoB 2.0) is an updated tool developed by the Cochrane Collaboration to assess the risk of bias in randomized controlled trials (RCTs). It is part of the Cochrane Handbook for Systematic Reviews of Interventions and is used to evaluate the methodological quality of studies included in systematic reviews.

Focus on Specific Domains

RoB 2.0 evaluates five key domains that could introduce bias in RCTs:

Randomization process: Assessing how participants were randomly assigned to groups.
Deviations from intended interventions: Considering whether participants received the interventions they were assigned to.
Missing outcome data: Evaluating how missing data were handled and its impact on results.
Measurement of the outcome: Checking if the outcome measurements were conducted in a way that minimized bias.
Selection of the reported result: Assessing whether the results reported were pre-specified and consistent with the planned analysis.

Rating each domain

Each domain is rated as:

Low risk of bias
Some concerns
High risk of bias

Purpose and Importance

The tool encourages reviewers to consider the context of the study and the potential impact of bias on the study's findings. It can be used for various types of outcomes (e.g., dichotomous, continuous) and different intervention types. By systematically assessing the risk of bias in included studies, reviewers can better determine the reliability of the findings and make informed conclusions about the effectiveness of interventions.