|1a||Systematic review (SR) of Randomised Clinical Trials (RCT)||
SR of Level 1 diagnostic studies;|
Clinical Decision Rule (CDR) with 1b studies from different clinical centres.
|1b||Individual RCT (with narrow confidence interval)||Validating cohort study with good reference standards, or CDR tested within one clinical centre.|
|1c||All or none††||
Diagnostic finding whose specificity is so high that a positive result rules-in the diagnosis.|
Diagnostic finding whose Sensitivity is so high that a Negative result rules-out the diagnosis.
|2a||SR of cohort studies||SR of Level 2a-c diagnostic studies|
|2b||Individual cohort study (including low-quality RCT; e.g., follow-up of <80% of patients)||
Exploratory cohort study with good reference standards;|
CDR after derivation, or validated only on split-sample or databases
|2c||Audits or "outcomes" research; Ecological studies|
|3a||SR of case-control studies||SR of Level 3b and better studies|
|3b||Individual Case-Control Study||Non-consecutive study; or one without consistently applied reference standards|
|4||Case-series (and poor-quality cohort and case-control studies)||Case-control study, poor or nonindependent reference standard|
|5||Expert opinion without explicit critical appraisal, or based on physiology, bench research or 'first principles'|