Validity of observational evidence on putative risk and protective factors: appraisal of 3744 meta-analyses on 57 topics

Table 1 The seven standardized criteria

Levels of evidence	Description
Convincing	• Associations with a statistical significance at P < 10⁻⁶ • More than 1000 cases included (or more than 20,000 participants for continuous outcomes) • The largest component study reporting a significant result at P < 0.05 • A 95% prediction interval that excluded the null • Absence of large heterogeneity (I2<50%) • No evidence of small study effects (P > 0.10) • No evidence of excess significance (P > 0.10)
Highly suggestive	• Associations with a statistical significance at P < 10⁻⁶ • More than 1000 cases included (or more than 20,000 participants for continuous outcomes) • The largest component study reporting a significant result at P < 0.05.
Suggestive	• Associations with a statistical significance at P < 0.001 • More than 1000 cases included (or more than 20,000 participants for continuous outcomes).
Weak	• Associations with a statistical significance at P < 0.05

Previous umbrella reviews have used various criteria to assess the evidence from meta-analysis of observational epidemiological studies. The combination of these criteria allows to tentatively classify evidence from meta-analyses of statistically significant risks and protective factors into four levels described below. A more detailed description of the criteria can be found in Additional file 1: Appendix Method 1

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