Evidence level

Quality criterion for a clinical study. The quality of clinical studies is classified in accordance with the AHRQ (Agency for Healthcare Research and Quality) in evidence classes / evidence levels from Ia to IV, whereby evidence class Ia is the highest quality.

Class Ia: evidence through at least one meta-analysis based on several methodologically high-quality randomised controlled studies

Class Ib: Evidence from at least one large, methodologically high-quality randomised study

Class IIa: evidence from at least one high-quality study without randomisation (non-randomised intervention studies)

Class IIb: evidence from several high-quality studies of a different type, quasi-experimental study

Class III: Evidence through more than one methodologically high-quality non-experimental study (e.g. non-interventional studies, prospective observational studies, case-control studies, correlation studies)

Class IV: Evidence based on reports / opinions of experts, consensus conferences and clinical experience of recognized authorities

An extension of the evidence hierarchy was made in the case of evidence levels according to the University of Oxford Levels of Evidence. This addresses the requirements of questions and evaluates weaknesses in the execution of the study.