Oxford Centre for Evidence-Based Medicine 2011 levels of evidence for diagnostic studies (modified according to ref 13)
| Level | Definition |
| 1 | Evidence from a systematic review of cross-sectional studies with consistently applied reference standard and blinding. |
| 2 | Individual cross-sectional studies with consistently applied reference standard and blinding. |
| 3 | Non-consecutive studies or studies without consistently applied reference standards. |
| 4 | Case–control studies or poor or non-independent reference standard. |
| 5 | Mechanism-based reasoning. |
Level of evidence may be downgraded based on study quality, imprecision, indirectness, because of inconsistency between studies or because the absolute effect size is very small. Level may be upgraded if there is a large or very large effect size.