Table 1.

Categories for strength of evidence and recommendations (Barnes et al., 2020; Shekelle et al., 1999).

Categories of evidence for causal relationships and treatment

Ia: Evidence from meta-analysis of RCTs

Ib: Evidence from at least one RCT

IIa: Evidence from at least one controlled study without randomisation

IIb: Evidence from at least one other type of quasi-experimental study

III: Evidence from non-experimental descriptive studies, such as comparative studies, correlation studies and case–control studies

IV: Evidence from expert committee reports or opinions and/or clinical experience of respected authorities

Categories of evidence for non-causal relationships

I: Evidence from large representative population samples

IIa: Evidence from small, well-designed, but not necessarily representative samples

IIb: Evidence from pharmacovigilance studies

III: Evidence from non-representative surveys, case reports

IV: Evidence from expert committee reports or opinions and/or clinical experience of respected authorities

Strength of recommendations

A: Directly based on category I evidence

B: Directly based on category II evidence or extrapolated recommendation from category I evidence

C: Directly based on category III evidence or extrapolated recommendation from category I or II evidence

D: Directly based on category IV evidence or extrapolated recommendation from category I, II or III evidence

S: Derived from a consensus view in the absence of systematic evidence

RCT: randomised controlled trial.