Table 1

Grading system to assess the level of evidence38

Strength of evidenceType of evidence
StrongConsistent findings in two or more studies of ++ quality
ModerateOne study or consistent findings in two or more + quality
LimitedOnly one + study, two or more studies with inconsistent findings
No evidenceNo study of acceptable quality (-), inconsistent findings or no relevant research available
  • ++, very low risk; +, low risk; −, high risk of confounding, bias or chance.