Table 1.

EBRO classification of the study results according to the level of evidence and definition of level of conclusion assigned per outcome

Level of evidence
A1	Systematic reviews and meta-analyses, based on minimally 2 independent A2 studies
A2	Prospective cohort studies with sufficient sample size and follow-up; adequately controlled for confounding factors and precluding selective loss to follow-up
B	Prospective cohort studies, but lacking the quality criteria of A2. Retrospective cohort studies and case–control studies
C	Non-comparative studies
D	Expert opinion
Level of conclusion
1	One A1 or at least two independent A2 studies that support each other’s conclusions and do not show conflicting evidence
2	One A2 or at least two independent B studies that support each other’s conclusions and do not show conflicting evidence
3	One B or at least two C studies that support each other’s conclusions and do not show conflicting evidence
4	Research at level C or two or more independent studies at higher level that do not support each other’s conclusions and show conflicting evidence
Consensus	There is no evidence, but consensus (level D)
No conclusions	There is no evidence and no consensus

EBRO, Evidence-Based Richtlijn (Guideline) Ontwikkeling (Development).