Table 1.

Level of evidence based on the Oxford Centre for Evidence-based Medicine (adapted from the Oxford 2011 Levels of Evidence)

Level	Criteria	Simple model for high, intermediate, and low evidence
1	SR (with homogeneity) of RCT	Further research is unlikely to change our confidence in the estimate of benefit and risk
2	RCT or observational studies with dramatic effects; SR of lower quality studies (i.e. non-randomised, retrospective)
3	Non-randomised controlled cohort/follow-up study/control arm of randomised trial (SR is generally better than an individual study)	Further research (if performed) is likely to have an impact on our confidence in the estimate of benefit and risk and may change the estimate
4	Case-series, case-control, or historically controlled studies (SR is generally better than an individual study)
5	Expert opinion (mechanism-based reasoning)	Any estimate of effect is uncertain

RCT: Randomised controlled trials; SR: Systematic reviews.