Classification of evidence levels
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 welldesigned
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 nonexperimental
descriptive studies, such as
comparative studies, correlation studies
and case studies.
IV Evidence obtained from expert committee
reports or opinions and/or clinical
experience of respected authorities