Table 1 Principles of evidence synthesis of systematic reviews

Evidence

 Consistent positive, significant effects on a specific outcome in at least one high quality systematic review (based on at least two underlying effect studies)

Limited evidence

 Effects on a specific outcome in at least one high quality systematic review (based on one underlying effect study)

Inconclusive evidence

 Inconsistent effects on a specific outcome, because at least one high quality systematic review (including at least two underlying studies) shows positive, significant effects, while other review(s) included did not find such effects.

No evidence

 None of the included reviews found consistent positive, significant effects on a specific outcome.

No research found

 None of the included reviews examined effects on a specific outcome.