Table 2.

Level of evidence based on primary research question, by Wright et al⁴⁰

	Therapeutic studies—investigating the results of treatment	Prognostic studies—investigating the outcome of disease	Diagnostic studies—investigating a diagnostic test
Level I	Good-quality randomised controlled trial, Systematic review of Level-I studies	Prospective study 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	Prospective cohort study Poor-quality randomised controlled trial Systematic review Level-II studies Nonhomogeneous level-I studies	Retrospective study Study of untreated controls from a previous randomised controlled trial, 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 cohort study Systematic review of level-III studies		Study of nonconsecutive patients (no consistently applied reference ‘gold’ standard) Systematic review of level-III studies
Level IV	Case series (with no, or historical, control group)	Case series	Case–control study Poor reference standard
Level V	Expert opinion	Expert opinion	Expert opinion