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).