Evidence classes / levels of evidence of scientific studies , represented as a pyramid. Studies can be from the fields of medicine, clinical studies, epidemiology and psychology. The hierarchical classification of studies is based on methodological aspects, according to the guidelines of the AHRQ (Agency for Healthcare Research and Quality, https://www.ahrq.gov). The classification of studies according to evidence levels is particularly suitable for clinical studies to assess the causality and effectiveness of interventions. Examples are therapy studies, evaluation of medication, training and other interventions. Hierarchy according to evidence level / evidence class Level Ia: Evidence based on at least one meta-analysis on the basis of several methodologically high-quality randomised controlled trials (RCTs) Level Ib: Evidence based on at least one large, methodologically high-quality randomised controlled trial (RCT) Level IIa: Evidence based on at least one methodologically high-quality study without randomised group allocation (non-randomised intervention study) Level IIb: Evidence based on at least one methodologically high-quality quasi-experimental study Level III: Evidence based on more than one methodologically high-quality non-experimental study (non-interventional study): e.g. case-control study, cohort study, cross-sectional study Level IV: Evidence based on reports / opinions of experts, based on clinical experience or expert committees Level V: Evidence based on case reports (single cases, few cases), case series (group, >10 cases) or individual expert opinions