Hierarchy of evidence

 

Categories of evidence

Ia	Evidence from a meta-analysis of randomised controlled trials.
Ib	Evidence from at least one randomised controlled trial.
IIa	Evidence from at least one controlled study without randomisation.
IIb	Evidence from at least one other type of quasi-experimental study.
III	Evidence from non-experimental descriptive studies, such as comparitive studies, correlation studies and case control studies.
IV	Evidence from expert committees’ reports or opinions and/or clinical experience of respected authorities

Strength of recommendation

A	Directly based on category I evidence.
B	Directly based on category II evidence or extrapolated recommendation from category I evidence.
C	Directly based on category III evidence or extrapolated recommendation from category I or II evidence.
D	Directly based on category IV evidence or extrapolated recommendation from category I, II or III evidence.