Text Box 1Hierarchy of evidence and recommendations grading scheme

LevelType of evidenceGradeEvidence
IEvidence obtained from a single randomised controlled trial or a meta-analysis of randomised controlled trialsAAt least one randomised controlled trial as part of a body of literature of overall good quality and consistency addressing the specific recommendation (evidence level I) without extrapolation
IIaEvidence obtained from at least one well-designed controlled study without randomisationBWell-conducted clinical studies but no randomised clinical trials on the topic of recommendation (evidence levels II or III); or extrapolated from level-I evidence
IIbEvidence obtained from at least one other well-designed quasi-experimental study
IIIEvidence obtained from well-designed non-experimental descriptive studies, such as comparative studies, correlation studies and case studies
IVEvidence obtained from expert committee reports or opinions and/or clinical experiences of respected authoritiesCExpert committee reports or opinions and/or clinical experiences of respected authorities (evidence level IV) or extrapolated from level I- or II-evidence.
This grading indicates that directly applicable clinical studies of good quality are absent or not readily available
GPPRecommended good practice based on the clinical experience of the GDG

Adapted from Eccles, M. and Mason, J. (2001) How to develop cost-conscious guidelines. Health Technology Assessment 5, 16; Mann, T. (1996) Clinical Guidelines: Using Clinical Guidelines to Improve Patient Care Within the NHS. London: Department of Health.


Cover of Obsessive-Compulsive Disorder
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Copyright © 2006, The British Psychological Society & The Royal College of Psychiatrists.

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