Table 1

Levels of evidence and grading of recommendations7

Level Type of evidence
IaMeta-analysis of randomised controlled trials
IbAt least one randomised controlled trial
IIaAt least one well designed controlled study without randomisation
IIbAt least one other type of well designed quasi-experimental study
IIIFrom well designed non-experimental descriptive studies
IVExpert committee reports of opinions and/or clinical experiences of respected authorities
Grade Type of evidence
A (levels Ia,Ib)Requires at least one randomised controlled trial
B (levels IIa, IIb, III)Well conducted clinical studies but no randomised controlled trial
C (level IV)Expert committee reports or opinion. Indicates absence of directly applicable studies of good quality