Table 1.

Evidence-based medicine levels of evidence[43]

Level	Therapy/prevention, etiology/harm	Prognosis
1a	SR (with homogeneity1) of RCTs	SR (with homogeneity1) of inception cohort studies; CDR2 validated in different populations
1b	Individual RCT (with narrow Confidence Interval2)	Individual inception cohort study with > 80% follow-up; CDR2 validated in a single population
1c	All or none3	All or none case-series
2a	SR (with homogeneity1) of cohort studies	SR (with homogeneity1) of either retrospective cohort studies or untreated control groups in RCTs
2b	Individual cohort study (including low quality RCT; e.g., < 80% follow-up)	Retrospective cohort study or follow-up of untreated control patients in an RCT; Derivation of CDR2 or validated on split-sample4 only
2c	"Outcomes" Research; Ecological studies	"Outcomes" research
3a	SR (with homogeneity1) of case-control studies
3b	Individual case-control study

¹A systematic review (SR) that is free of worrisome variations (heterogeneity) in the directions and degrees of results between individual studies. Studies displaying worrisome heterogeneity should be tagged with a “-” at the end of their designated level;

²Clinical decision rule (CDR) (These are algorithms or scoring systems that lead to a prognostic estimation or a diagnostic category);

³Met when all patients died before the Rx became available, but some now survive on it; or when some patients died before the Rx became available, but now none die on it;

⁴Split-sample validation is achieved by collecting all the information in a single tranche, and subsequently artificially dividing this into “derivation” and “validation” samples. RCTs: Randomized control trials.