Classification of methodological quality of individual studies
| Classification of studies | Intervention | Diagnostic accuracy of research | Damage or side effects, aetiology, prognosis* |
| A1 | Systematic review of at least two independently conducted studies of A2 level | ||
| A2 | Randomised double-blind comparative clinical research of good quality and sufficient sample size | Research relative to a reference test (a ‘golden standard’) with predefined cut-off points and independent assessment of the results of a test, on a sufficiently large series of consecutive patients who all have had the index and reference test | Prospective cohort study of sufficient sample size and follow-up duration adequately controlled for ‘confounding’ and selective follow-up is sufficiently excluded |
| B | Comparative research, but not with all the features as mentioned under A2 (this includes patient control research, cohort study) | Research relative to a reference test, but not with all the attributes that are listed under A2 | Prospective cohort study, but not with all the features as mentioned under A2 or retrospective cohort study or patient monitoring research |
| C | Not comparative research | ||
| D | Opinion of experts |
*This classification only applies to situations in which due to ethical or other reasons controlled trials are not possible to perform. If these are possible, then the classification applies to interventions.