Table 2

Evidence level

Evidence level
High quality	One or more well-designed and well-executed randomised controlled trials (RCTs) that yield consistent and directly applicable results This level also means that further research is very unlikely to change our confidence in the estimate of effect
Moderate quality	RCTs with important limitations (ie, biased assessment of the treatment effect, large loss to follow-up, lack of blinding, unexplained heterogeneity), indirect evidence originating from similar (but not identical) populations of interest and RCTs with very small numbers of participants or observed events In addition, evidence from well-designed controlled trials without randomisation, from well-designed cohort or case-controlled analytical studies, and from multiple time series with or without intervention is included in this category This level also means that further research will probably have an important impact on our confidence in the estimate of effect and may change the estimate
Low quality	Observational studies would typically be rated as low quality because of the risk for bias* This level also means that further research is very likely to have an important impact on our confidence in the estimate of effect and will probably change the estimate
Very low quality†	Evidence is conflicting, of poor quality or lacking, and hence the balance of benefits and harms cannot be determined Any estimate of effect is very uncertain as evidence is either unavailable or does not permit a conclusion

*Quality of evidence based on observational studies may be rated as moderate or even high, depending on circumstances under which evidence is obtained from observational studies. Factors that may contribute to upgrading the quality of evidence include a large magnitude of the observed effect, a dose–response association or the presence of an observed effect when all plausible confounders would decrease the observed effect.
†Insufficient evidence to determine for or against routinely providing a service.