Levels of Evidence
From the Centre for Evidence-Based Medicine, Oxford
For the most up-to-date levels of evidence, see http://www.cebm.net/levels_of_evidence.asp
Therapy/Prevention/Etiology/Harm:
| 1a: | Systematic reviews (with homogeneity ) of randomized controlled trials |
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| 1a-: | Systematic review of randomized trials displaying worrisome heterogeneity |
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| 1b: | Individual randomized controlled trials (with narrow confidence interval) |
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| 1b-: | Individual randomized controlled trials (with a wide confidence interval) |
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| 1c: | All or none randomized controlled trials |
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| 2a: | Systematic reviews (with homogeneity) of cohort studies |
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| 2a-: | Systematic reviews of cohort studies displaying worrisome heterogeneity |
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| 2b: | Individual cohort study or low quality randomized controlled trials (<80% follow-up) |
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| 2b-: | Individual cohort study or low quality randomized controlled trials (<80% follow-up / wide confidence interval) |
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| 2c: | 'Outcomes' Research; ecological studies |
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| 3a: | Systematic review (with homogeneity) of case-control studies |
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| 3a-: | Systematic review of case-control studies with worrisome heterogeneity |
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| 3b: | Individual case-control study |
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| 4: | Case-series (and poor quality cohort and case-control studies) |
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| 5: | Expert opinion without explicit critical appraisal, or based on physiology, bench research or 'first principles' |
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Diagnosis:
| 1a: | Systematic review (with homogeneity) of Level 1 diagnostic studies; or a clinical rule validated on a test set. |
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| 1a-: | Systematic review of Level 1 diagnostic studies displaying worrisome heterogeneity |
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| 1b: | Independent blind comparison of an appropriate spectrum of consecutive patients, all of whom have undergone both the diagnostic test and the reference standard; or a clinical decision rule not validated on a second set of patients |
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| 1c: | Absolute SpPins And SnNouts (An Absolute SpPin is a diagnostic finding whose Specificity is so high that a Positive result rules-in the diagnosis. An Absolute SnNout is a diagnostic finding whose Sensitivity is so high that a Negative result rules-out the diagnosis). |
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| 2a: | Systematic review (with homogeneity) of Level >2 diagnostic studies |
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| 2a-: | Systematic review of Level >2 diagnostic studies displaying worrisome heterogeneity |
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| 2b: | Any of: 1)independent blind or objective comparison; 2)study performed in a set of non-consecutive patients, or confined to a narrow spectrum of study individuals (or both) all of whom have undergone both the diagnostic test and the reference standard; 3) a diagnostic clinical rule not validated in a test set. |
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| 3a: | Systematic review (with homogeneity) of case-control studies |
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| 3a-: | Systematic review of case-control studies displaying worrisome heterogeneity |
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| 4: | Any of: 1)reference standard was unobjective, unblinded or not independent; 2) positive and negative tests were verified using separate reference standards; 3) study was performed in an inappropriate spectrum of patients. |
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| 5: | Expert opinion without explicit critical appraisal, or based on physiology, bench research or 'first principles' |
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Prognosis:
| 1a: | Systematic review (with homogeneity) of inception cohort studies; or a clinical rule validated on a test set. |
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| 1a-: | Systematic review of inception cohort studies displaying worrisome heterogeneity |
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| 1b: | Individual inception cohort study with > 80% follow-up; or a clinical rule not validated on a second set of patients |
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| 1c: | All or none case-series |
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| 2a: | Systematic review (with homogeneity) of either retrospective cohort studies or untreated control groups in RCTs. |
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| 2a-: | Systematic review of either retrospective cohort studies or untreated control groups in RCTs displaying worrisome heterogeneity |
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| 2b: | Retrospective cohort study or follow-up of untreated control patients in an RCT; or clinical rule not validated in a test set. |
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| 2c: | 'Outcomes' research |
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| 4: | Case-series (and poor quality prognostic cohort studies) |
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| 5: | Expert opinion without explicit critical appraisal, or based on physiology, bench research or 'first principles' |
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Key to interpretation of practice guidelines
Agency for Healthcare Research and Quality:
| A: | There is good research-based evidence to support the recommendation. |
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| B: | There is fair research-based evidence to support the recommendation. |
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| C: | The recommendation is based on expert opinion and panel consensus. |
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| X: | There is evidence of harm from this intervention. |
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USPSTF Guide to Clinical Preventive Services:
| A: | There is good evidence to support the recommendation that the condition be specifically considered in a periodic health examination. |
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| B: | There is fair evidence to support the recommendation that the condition be specifically considered in a periodic health examination. |
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| C: | There is insufficient evidence to recommend for or against the inclusion of the condition in a periodic health examination, but recommendations may be made on other grounds. |
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| D: | There is fair evidence to support the recommendation that the condition be excluded from consideration in a periodic health examination. |
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| E: | There is good evidence to support the recommendation that the condition be excluded from consideration in a periodic health examination. |
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University of Michigan Practice Guideline:
| A: | Randomized controlled trials. |
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| B: | Controlled trials, no randomization. |
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| C: | Observational trials. |
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| D: | Opinion of the expert panel. |
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Other guidelines:
| A: | There is good research-based evidence to support the recommendation. |
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| B: | There is fair research-based evidence to support the recommendation. |
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| C: | The recommendation is based on expert opinion and panel consensus. |
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| X: | There is evidence that the intervention is harmful. |
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GRADE (Grading of Recommendations Assessment, Development and Evaluation)
| Code |
Quality of Evidence |
Definition |
| A |
High |
Further research is very unlikely to change our confidence in the estimate of effect.
- Several high-quality studies with consistent results
- In special cases: one large, high-quality multi-centre trial
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| B |
Moderate |
Further research is likely to have an important impact on our confidence in the estimate of effect and may change the estimate.
- One high-quality study
- Several studies with some limitations
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| C |
Low |
Further research is very likely to have an important impact on our confidence in the estimate of effect and is likely to change the estimate.
- One or more studies with severe limitations
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| D |
Very Low |
Any estimate of effect is very uncertain.
- Expert opinion
- No direct research evidence
- One or more studies with very severe limitations
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Source: GRADE (Grading of Recommendations Assessment, Development and Evaluation) Working Group 2007 1 (modified by the EBM Guidelines Editorial Team)
