Levels of Evidence

Hierarchical systems for levels of evidence and recommendations

    * A variety of grading systems for evidence and recommendations are currently in use. The system used is usually defined at the beginning of any guidelines publication.
    * The hierarchy of evidence and the recommendation grading's relate to the strength of the literature and not necessarily to clinical importance.

Grading of evidence

    * Ia: systematic review or meta-analysis of randomised controlled trials
    * Ib: at least one randomised controlled trial
    * IIa: at least one well-designed controlled study without randomisation
    * IIb: at least one well-designed quasi-experimental study, such as a cohort study
    * III: well-designed non-experimental descriptive studies, such as comparative studies, correlation studies, casecontrol studies and case series
    * IV: expert committee reports, opinions and/or clinical experience of respected authorities

Grading of recommendations

    * A: based on hierarchy I evidence
    * B: based on hierarchy II evidence or extrapolated from hierarchy I evidence
    * C: based on hierarchy III evidence or extrapolated from hierarchy I or II evidence
    * D: directly based on hierarchy IV evidence or extrapolated from hierarchy I, II or III evidence

A simpler system of A,B or C is recommended by the US Government Agency for Health Care Policy and Research (AHCPR):

    * A: requires at least one randomised controlled trial as part of the body of evidence.
    * B: requires availability of well-conducted clinical studies but no randomised controlled trials in the body of evidence.
    * C: requires evidence from expert committee reports or opinions and/ or clinical experience of respected authorities. Indicates absence of directly applicable studies of good quality.

Guideline recommendation and evidence grading (GREG)

In an attempt to improve the way recommendations and evidence statements are graded, a new grading system has been used (Guideline Recommendation and Evidence Grading - GREG):

    * Evidence grade:
          o I (High): the described effect is plausible, precisely quantified and not vulnerable to bias
          o II (Intermediate): the described effect is plausible but is not quantified precisely or may be vulnerable to bias
          o III (Low): concerns about plausibility or vulnerability to bias severely limit the value of the effect being described and quantified
    * Recommendation grade:
          o A (Recommendation): there is robust evidence to recommend a pattern of care
          o B (Provisional recommendation): on balance of evidence, a pattern of care is recommended with caution
          o C (Consensus opinion): evidence being inadequate, a pattern of care is recommended by consensus

Exerpt from Patient UK

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