Table 1

Levels of evidence and grading of recommendations based on the US Agency for Health Care Policy and Research (AHCPR)10

Level Type of evidence
IaEvidence obtained from meta-analysis of randomised controlled trials
IbEvidence obtained from at least one randomised controlled trial
IIaEvidence obtained from at least one well designed controlled study without randomisation
IIbEvidence obtained from at least one other type of well designed quasi-experimental study
IIIEvidence obtained from well designed non-experimental descriptive studies such as comparative studies, correlation studies, and case controlled studies
IVEvidence obtained from expert committee reports of opinions and/or clinical experiences of respected authorities
Grade Type of recommendations
A (levels Ia, Ib)Requires at least one randomised controlled trial as part of a body of literature of overall good quality and consistency addressing the specific recommendation
B (levels IIa, IIb, III)Requires availability of well conducted clinical studies but no randomised clinical trials on the topic of recommendation
C (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