Table 2.

Grades of Evidence.

	Therapeutic Study	Prognostic Study	Diagnostic Study
Level I	• High-quality RCT with statistically significant difference or no statistically significant difference but narrow confidence intervals • Systematic review of Level I RCT (and study results were homogeneous)	• High-quality prospective trial (all patients were enrolled at the same point in their disease with > 80% follow-up of enrolled patients) • Systematic review of Level I studies	• Testing of previously developed diagnostic criteria in series of consecutive patients (with universally applied reference “gold” standard) • Systematic review of Level I studies
Level II	• Lesser quality RCT (<80% follow-up, no blinding, or improper randomization) • Prospective comparative study • Systematic review of Level II studies or Level I studies with inconsistent results	• Retrospective study • Untreated controls from an RCT • Lesser quality prospective study (patients enrolled at different points in their disease or <80% follow-up) • Systematic review of Level II studies	• Development of diagnostic criteria on basis of consecutive patients (with universally applied reference “gold” standard) • Systematic review of Level II studies
Level III	• Case-control study • Retrospective comparative study • Systematic review of Level III studies	• Case-control study	• Study of nonconsecutive patients (without consistently applied reference “gold” standard) • Systematic review of Level III studies
Level IV	• Case series	• Case series	• Case-control study • Poor reference standard
Level V	• Expert opinion	• Expert opinion	• Expert opinion

RCT = Randomized Controlled Trial.