Table 2.

Levels of evidence for prognostic studies (adapted from Wupperman et al. [24])

Level of evidence	Study characteristics
I	High-quality prospective studya (all patients were enrolled at the same point in their disease with ≥80 % follow-up of enrolled patients
	Systematic reviewb of level I studies
II	Retrospectivec study
	Untreated controls from an RCT
	Lesser quality prospective study (e.g., patients enrolled at different points in their disease or <80 % follow-up)
	Systematic review of level II studies
III	Case control studyd
IV	Case series
V	Expert opinion

^aStudy was started before the first patient enrolled

^bA combination of results from two or more prior studies

^cStudy was started after the first patient enrolled

^dPatients identified for the study based on their outcome, called ‘cases’ are compared to those who did not have that outcome, called ‘controls’