Table 1 Rating system for risk of bias of individual studies

Level 1

∙ Randomised controlled trials with Low risk of bias as assessed by RoB 2.0

Level 2

∙ Randomised controlled trials with Some concerns or High risk of bias as assessed by RoB 2.0

∙ Non-randomised controlled trials (including case–control, prospective and retrospective cohort studies) with Low or Moderate risk of bias as assessed by ROBINS-I

Level 3

∙ Non-randomised controlled trials (including case–control, prospective and retrospective cohort studies) with Serious or Critical risk of bias as assessed by ROBINS-I

∙ Pre-post studies (any Downs and Black score)

Level 4

∙ Case series defined by 3–5 individuals only (any Downs and Black score)

∙ Case report defined by 1–2 individuals only (any Downs and Black score)

∙ Cross-sectional controlled study (any Downs and Black score)