Table 1

Levels of evidence according to criteria from Centre for Evidence Based Medicine, Oxford

1aSystematic review of level 1 studies with homogeneity
1bProspective cohort study with good reference standards
1cAbsolutely specific or sensitive
2aSystematic review of level 2 or >2 studies with homogeneity
2bExploratory cohort study with good reference standards
3aSystematic review of studies from level 1, 2, or 3b
3bStudy of non-consecutive cases or without universally applied reference standard
4Case-control study with poor or non-independent reference standard
5Expert opinion or based on physiology, bench research, or first principles