Levels of evidence
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In medicine, levels of evidence (LoE) are arranged in a ranking system used in evidence-based practices to describe the strength of the results measured in a clinical trial or research study. The design of the study (such as a case report for an individual patient or a double-blinded randomized controlled trial) and the endpoints measured (such as survival or quality of life) affect the strength of the evidence.
The National Cancer Institute defines levels of evidence as "a ranking system used to describe the strength of the results measured in a clinical trial or research study. The design of the study [...] and the endpoints measured [...] affect the strength of the evidence."
The term was first used in a 1979 report by the "Canadian Task Force on the Periodic Health Examination" (CTF) to "grade the effectiveness of an intervention according to the quality of evidence obtained".:1195 The task force used three levels, subdividing level II:
- Level I: Evidence from at least one randomized controlled trial,
- Level II1: Evidence from at least one well designed cohort study or case control study, i.e. a controlled trial which is not randomized
- Level II2: Comparisons between times and places with or without the intervention
- Level III: Opinions of respected authorities, based on clinical experience, descriptive studies or reports of expert committees.
The CTF graded their recommendations into a 5-point A–E scale: A: Good level of evidence for the recommendation to consider a condition, B: Fair level of evidence for the recommendation to consider a condition, C: Poor level of evidence for the recommendation to consider a condition, D: Fair level evidence for the recommendation to exclude the condition, and E: Good level of evidence for the recommendation to exclude condition from consideration.:1195 The CTF updated their report in 1984, in 1986 and 1987.
- Level I: Evidence obtained from at least one properly designed randomized controlled trial.
- Level II-1: Evidence obtained from well-designed controlled trials without randomization.
- Level II-2: Evidence obtained from well-designed cohort or case-control analytic studies, preferably from more than one center or research group.
- Level II-3: Evidence obtained from multiple time series designs with or without the intervention. Dramatic results in uncontrolled trials might also be regarded as this type of evidence.
- Level III: Opinions of respected authorities, based on clinical experience, descriptive studies, or reports of expert committees.
Over the years many more grading systems have been described.
In September 2000, the Oxford (UK) CEBM Levels of Evidence published its guidelines for 'Levels' of evidence re claims about prognosis, diagnosis, treatment benefits, treatment harms, and screening. It not only addressed therapy and prevention, but also diagnostic tests, prognostic markers, or harm. The original CEBM Levels was first released for Evidence-Based On Call to make the process of finding evidence feasible and its results explicit. As published in 2009 they are:
- 1a: Systematic reviews (with homogeneity) of randomized controlled trials
- 1b: Individual randomized controlled trials (with narrow confidence interval)
- 1c: All or none randomized controlled trials
- 2a: Systematic reviews (with homogeneity) of cohort studies
- 2b: Individual cohort study or low quality randomized controlled trials (e.g. <80% follow-up)
- 2c: "Outcomes" Research; ecological studies
- 3a: Systematic review (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"
In 2011, an international team redesigned the Oxford CEBM Levels to make it more understandable and to take into account recent developments in evidence ranking schemes. The Levels have been used by patients, clinicians and also to develop clinical guidelines including recommendations for the optimal use of phototherapy and topical therapy in psoriasis and guidelines for the use of the BCLC staging system for diagnosing and monitoring hepatocellular carcinoma in Canada.
In 2007, the World Cancer Research Fund grading system described 4 levels: Convincing, probable, possible and insufficient evidence. All Global Burden of Disease Studies have used it to evaluate epidemiologic evidence supporting causal relationships.
The hierarchy of evidence produced by a study design has been questioned, because guidelines have "failed to properly define key terms, weight the merits of certain non-randomized controlled trials, and employ a comprehensive list of study design limitations".
Stegenga has criticized specifically that meta-analyses are placed at the top of such hierarchies. The assumption that RCTs ought to be necessarily near the top of such hierarchies has been criticized by Worrall. and Cartwright
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- Evidence levels with explanations – entry in the Centre for Evidence-Based Medicine
- Evidence-based medicine resources page – with a diagram showing different levels of evidence forming a pyramid