Table 1. Evidence levels Evidence level... | American Society of Hematology

Table 1.

Evidence levels

Evidence level	Definition
Ia	Evidence obtained from meta-analysis of RCTs
Ib	Evidence obtained from ≥1 RCT
IIa	Evidence obtained from ≥1 well-designed controlled study without randomization
IIb	Evidence obtained from ≥1 other type of well-designed quasi-experimental study*
III	Evidence obtained from well-designed nonexperimental descriptive studies, such as comparative studies, correlated studies, and case studies
IV	Evidence obtained from expert committee reports or opinions and/or clinical experience of respected authorities

Evidence level	Definition
Ia	Evidence obtained from meta-analysis of RCTs
Ib	Evidence obtained from ≥1 RCT
IIa	Evidence obtained from ≥1 well-designed controlled study without randomization
IIb	Evidence obtained from ≥1 other type of well-designed quasi-experimental study*
III	Evidence obtained from well-designed nonexperimental descriptive studies, such as comparative studies, correlated studies, and case studies
IV	Evidence obtained from expert committee reports or opinions and/or clinical experience of respected authorities

Refers to a situation in which implementation of an intervention is not under the control of the investigators, but an opportunity exists to evaluate its effect.