Key 2—OCEBM Level of Evidence (adapted from www.cebm.net)

Level	Therapy/prevention, aetiology/harm
1a	Systematic review (SR) (with homogeneity) of randomized controlled trials (RCTs)
1b	Individual RCT (with narrow confidence interval)
1c	All or none
2a	SR (with homogeneity) of cohort studies
2b	Individual cohort study (including low quality RCT; e.g. <80% follow-up)
2c	‘Outcomes’ research; ecological studies
3a	SR (with homogeneity) of case–control studies
3b	Individual case–control study
4	Case series (and poor quality cohort and case–control studies)
5	Expert opinion without explicit critical appraisal, or based on physiology, bench research or ‘first principles’
Others	For example, model simulations, non-human-based experiment, in vitro or in situ studies, and statistical reports or dashboards