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Levels of Evidence
From US Agency for Health Care Policy and Research (AHCPR), now known as the Agency for Healthcare Research and Quality (AHRQ). Used by SIGN (Scottish Intercollegiate Guidelines Network) & North of England evidence based guidelines development project.
| Level |
Type of evidence (based on AHCPR 1992) |
| Ia |
Evidence obtained from meta-analysis of randomised controlled trials |
| Ib |
Evidence obtained from at least one randomised controlled trial |
| IIa |
Evidence obtained from at least one well-designed controlled study without randomisation |
| IIb |
Evidence obtained from at least one other type of well-designed quasi-experimental study |
| III |
Evidence obtained from well-designed non-experimental descriptive studies, such as comparative studies, correlation studies and case control studies |
| IV |
Evidence obtained from expert committee reports or opinions and/or clinical experience of respected authorities |
Grading of recommendations
| Grade |
Recommendation (based on AHCPR 1994) |
A
(Evidence levels Ia, Ib) |
Requires at least one randomised controlled trial as part of the body of literature of overall good quality and consistency addressing the specific recommendation |
B
(Evidence levels IIa, IIb, III) |
Requires availability of well-conducted clinical studies but no randomised clinical trials on the topic of recommendation |
C
(Evidence level IV) |
Requires evidence from expert committee reports or opinions and/or clinical experience of respected authorities. Indicates absence of directly applicable studies of good quality | |
